AO: Animation Over-rider. It's something either part of a browser or in an object you wear which makes your avatar move in a way different from the default moves.
Alt: Alternate accounts in Second Lie (and elsewhere).
Avatar: The form your account takes in Second Life. The basic avatars are humanoid, but many stores sell avatars in other shapes, form mundane animals to fantastical beasts.
Bling: A specific script, usually put in jewelry, which causes flashes of white to show up around an item. Bling has a certain amount of controversy, often critiqued as being crass or unattractive.
Building: Taking shapes, called prims (short for primitives) and combining them to make things.
Chat: Communicating with other people through text, either in the immediate surrounding or through long distance or group communication.
Child Avatars: Small avatars shaped to look like children. Short avatars are sometimes mistaken for these. Child Avatars may be banned from set Mature and Adult sims due to fears of their being exposed or included in sexual things. Some child avatars get families or speak in their own language of a kind which imitates lisps and primitive English grammar.
Click, Click On: Put the cursor on an object and click the mouse like you are clicking on a web link to open it (also called touch).
Emote: Communicate something in text as if you are doing it, for example: type '/me dances.' to shares [Your Username] dances.
Flexi: Flexible. It's a setting you can set on an item which will make it bend.
Gesture: A pre-set series of scripts, text, sounds, and animations that are started by a trigger word. There are some inherent to Second Life's programming, or you can purchase them.
Glow: A setting on prims which makes them appear to (shocking) glow.
Group: Something you join which allows for chat, distribution of notecards, sharing of land, land-rights, titles over your name, and other perks. You have up to 42 group slots in Second Life. You may be charged Linden to join a group.
Home: A point where you get sent back to when killed (only possible on some grids) and where you can return too easily. You can set them in certain sims.
Hovertext: Text which hovers over an object, initially caused by a script. It remains even after the script is removed, so the script has to be reset to not have any text before removing it.
Inventory: Items you own; can include anything created in Second Life, including things which seem counter-intuitive like houses and pets.
Lucky Chair/Board: A chair or board you can find which shows a number or letter of the alphabet. When the first letter/number of your name shows up, you click it or sit on it and you win the item.
Main: Main account in Second Life (and elsewhere).
Mesh: A different way of uploading content which is more in line with content in other video games. Mesh can be in a rezzed (see below) object or worn. When worn, there is a setting called "rigged" which connects the size and shape of the mesh object to the skeleton of your avatar. This has been used to make much larger and much smaller avatars, as well as more realistic styles of clothing. These are made in an outside program, like blender.
Midnight Mania: Midnight Manias are a sales and traffic driver for retail stores, whereby if the customers can get a number of people to click a board in a day, then everyone gets something for free.
Mini-mania: Like a midnight mania but not time dependent, where a much lower number of people need to click (usually 5-10 range) to get a chance at winning the item, one person per round.
Permissions: The rights a user has with an object. All prims (see below) you make are full permission to yourself, and have auto-set permissions to anyone you hand them too. The permissions are: copy, modify (mod), transfer (trans). Most creators make something either copy or transfer, not both; modify permissions are much more idiosyncratic. Full Perm means you have full permission to copy, modify, or transfer something. Permissions are also what you are allowed to do on a sim, like create things or run scripts.
Petites: A form of mesh (see above) avatar that is about 1/3 the size of a standard avatar. A couple of other specifically smaller avatars have been made, like Minikins.
Prims: Primitives - a single item or shape. Used to build things.
RP: Role Playing. Where you dress up your avatar and act out roleplaying, often in a sim partly or wholly dedicated to the roleplaying universe, complete with buildings and stores. Sub-terms- IC (In Character), OOC (Out Of Character).
Script: A program which causes things to happen. It has it's own programming language and more information can be found here.
Sculpty: A primitive in a non-basic shape. Sculpted prims, or sculpties are usually made in an external program, like Blender.
Tinies: Small avatars, often animal shapes and much, much smaller than the usual avatar. Tinies can have their own culture and gathering places.
Rez: 1) To appear in the world as an avatar (most people have a "rez date" which is the date of their account creation); 2) To cause something to appear in your environment by dragging it from your inventory onto the ground of a place with building permissions; 3) The process of the things around you moving from a gray, unsculpted form to the final, textured form.
Sim: Short for Simulator, it indicates the land you're standing on. Sims can be linked up to each other to make larger land.
Windlight: A lighting system for Second Life which allows a person to modify most of the aspects of their surrounding in terms of lighting and the appearance of the sky and water.