Forum-related:
Sig - an image that is displayed a user's signature area, the content added at the end of each post from that user
Avatar - an image, usually around 80x80 pixels on most forums, that is placed below the user's display name.
Post - a message on the forums.
Thread - a group of messages on a forum usually with a specific topic.
Category - a group of threads with a specific topic, such as "GIMP General Help", used to get help with using GIMP.
Moderator - a user with special privileges such as editing or deleting anyone's posts, moving or splitting threads, and various other things normal users can't or are restricted to (users can edit their own posts, but not others' posts). They help keep the forums in good shape (removing spam, illegal content, and other things that don't obey the forum rules).
Administrator - a user with more privileges than a moderator in that they have full control over the forums, such as the creation of new categories, assigning moderators.
General image-creation:
RGB - an image mode, named after the 3 channels it uses, red, green, and blue, that is often called "true color".
grayscale - an image mode where only shades of gray are used.
indexed - an image mode that uses a palette for colors.
alpha channel - a channel, much like red, green, and blue, which is often used for transparency. It ranges from 0 to 255 (0 to 100%) where 0 is fully transparent and 255 is fully opaque.
layer - a separate image that makes up a larger image, drawn bottom up (higher layers are drawn on top of the next lower layer).
palette - a group of colors, often used for indexed mode or frequently used colors.
channel - a group of 8-bit values used to compose an image. RGB mode uses 3 main channels - red, green, and blue, but sometimes alpha. Channels can also be of selections.
gradient - a blend of multiple colors from one color to another. The basic gradient involves the foreground color fading to the background color. That is, red fading to yellow would have orange about halfway between and golden-yellow close to the yellow.
canvas size - the size of the image, usually in pixels.
physical size - the size of the image in physical units such as inches or centimeters, dependent on the image resolution.
Animation-related:
frame - a still image that makes up an animation or video.
fps - the number of frames rendered per second (stands for "frames per second").
File formats:
BMP (BitMaP) - a file format supporting 1, 4, 8, or 24-bit color, and RLE compression (except for 24-bit color images).
GIF (Graphics Interchange Format) - a file format, common on the internet, that uses a palette and supports up to 8-bit color and monochromatic transparency (either fully opaque or fully transparent). GIFs can also be animated.
JPG - a file format, common on the internet and with digital photographs, that uses lossy compression.
PNG - a file format, common on the internet, that uses lossless compression and supports transparency and up to 64-bit color.
TGA - a file format, often used with games, that supports RLE compression and transparency (32-bit color).
XCF - GIMP's native file format, which preserves almost everything (all layers, channels, selections - only undo history is not preserved).
General image-related content:
RLE compression - RLE, standing for "run-length encoded", is a lossless file compression method which involves repeating a set amount of data. For example, the notation "2D 01 20 40 80 FF" means that the HTML color 804020 (a brownish color) at full opacity is repeated 300 times. The first part is the number of times to repeat something (0 for special things) and that after it is what is repeated.
lossy compression - a compression method that alters the data, such as with JPG or MP3 (or almost all video codecs), where the alterations are minimal to maximize compression. Repeatedly saving a lossy compressed image gradually deteriorates the quality.
lossless compression - a compression method that does not modify the data to enhance compression, such as GIF and PNG.
dithering - the concept of blending two different colors together to simulate another color, often used to get more colors for a smaller palette.
Other GIMP-related content:
Scheme - the programming language that GIMP uses to automate specific tasks, whether loading a series of files for an animation, or creating a random-stone texture. (please clarify)
Histogram - a graph that shows the amount of values over the entire layer.
For FAQ questions:
Q. How do I get an avatar below my name?
A. First, upload your image to some image-hosting area like imageshack, photobucket, etc.. The image needs to be 100x100 pixels and can be of the GIF, JPG, or PNG format. Then, once uploaded, copy the URL you get, click on "account settings" at the top, insert your password where indicated in order to update the profile, then scroll about halfway down to where you see "avatar". Paste the URL into here and click the "update information" button. To check the image, view one of your own posts and you should see it (refresh the page if needed).
Q. I've uploaded an image, but I get something else instead of it. Why?
A. Some webhosts and image-hosting services, such as Angelfire, do not support hotlinking to images. Hotlinking is directly linking to non-webpage files, usually images, from another unrelated site. To resolve this, you must upload it to a site or service that supports hotlinking, such as your own paid webhost or image-hosting sites like imageshack or photobucket. With the URL now available, you should be able to display the image.
Q. How do I do a clean install of GIMP?
A. A clean install is an installation made as if the program was never installed before, even though it was. This is useful for troubleshooting or reverting back to an older, otherwise more stable version. The method depends on the operating system. The steps marked as optional don't have to be done, but a cleaner install is possible when all optional steps are done.
For Windows:
1. Close GIMP completely (use Windows Task Manager (press control+alt+delete to bring it up) to terminate any gimp-2.2.exe and scriptfu.exe processes you find.
2. Access the Windows control panel and go to add/remove programs.
3. (Optional) Download the version you want, usually the newest version (accessible
here).
4. Remove The GIMP following the dialogs and the typical uninstall procedure.
5. Open Windows Explorer and go to where you installed GIMP. This is usually "C:\Program Files\GIMP-2.2\". If you added any scripts, brushes, etc. here instead of the dedicated user folder, you may want to back these up (copy them to another temporary folder). Delete this entire folder after having backed up what you wanted.
6. (Optional) Browse to your user folder (usually C:\Documents and Settings\##username##\gimp-2.2\) and back up any content, as done in step 4 (though a different folder would be better). Delete all files here.
7. (Optional) Restart your computer.
8. Install GIMP following the dialogs.
For Linux:
(please expand on this - it should be somewhat similar to the Windows method involving the deleting and uninstalling but I have no experience with Linux)
For Macintosh:
(see the above for Linux)
Q. How do I do a clean install of the GTK+?
A. Repeat the same procedure for a clean install of The GIMP, except the install folder to be deleted is located at C:\Program Files\common files\GTK\ and there shouldn't be any user-related files to backup and delete.
Q. What is the GTK+ for?
A. The GTK+ (sometimes referred to as just "GTK") is used for Windows as it contains Windows-related functions such as those for the menus or the displaying of the image in the image window. (clarification and accuracy should be verified, I'm only 90% certain)
As a suggestion, for the glossary anyway, perhaps a dictionary-like format can be used, and arranged alphabetically? My site does that for my commonly used terms. For the FAQ, I would think they could be arranged based first on subject (such as one subject for tools, another for animation, etc.), likely arranged alphabetically, then by topic, which then contains the information. For tutorials, something similar can be done as with the FAQ except that it contains the difficulty (beginner, advanced, expert, and master).
Edit: added two FAQ questions, created "categories" for my glossary entries, added many other forum-related glossary entries.
Edit #2: Scheme, palette, channel, histogram, gradient, canvas size, and physical size added to glossary.
Edit #3: Added 3 new FAQ questions - 2 for clean installs and one for what the GTK+ is.