Alpha

Borrowed from traditional (i.e. non-games) software development, Alpha designates a key stage in a game's development. The definition of Alpha for traditional software is, according to Wikipedia:

Alpha software can be unstable and could cause crashes or data loss. [...] The alpha phase usually ends with a feature freeze, indicating that no more features will be added to the software. At this time, the software is said to be feature complete.

In game development, the definition is a bit more blurry.

Gamasutra - Features - The Cake Is Not a Lie: How to Design Effective Achievements

Gamasutra - Features - The Cake Is Not a Lie: How to Design Effective Achievements

Pillars

Pillars, as the name suggests, are the foundations of a game. People might find that this is basically the same thing as Unique Selling Points (sometimes Key Selling Points), but it doesn't encompass exactly the same things.

Gamasutra - Features - First-Person Control Design for Dual Analog Stick Controllers

An old article I've found on some forum. Good read.

Gamasutra - Features - First-Person Control Design for Dual Analog Stick Controllers

Unique Selling Point

Unique Selling Points, USPs (or sometimes called Key Selling Points) are the elements that make your game stand out, what makes it better than and/or different than titles from the competition. Note the "Selling" part. These points are supposed to make the players want to buy the game. Or at the very least, make the publisher want to invest in that project :)

Mission statement

While a fairly self explanatory term, the mission statement is used slightly differently in a game's design. According to Wikipedia:
"A mission statement is a formal, short, written statement of the purpose of a company or organization. The mission statement should guide the actions of the organization, spell out its overall goal, provide a sense of direction, and guide decision-making."

Gating

Gating is a level design technique that consists in confining the player to a small area and/or blocking the access to another area, until the player completes a given objective or makes a specific action. There are several kinds of gating, used in different situations.

Aim systems in First Person Shooters

Earlier this week, FPS-focused (French) website NoFrag published a very interesting article, making a quite thorough analysis of the various aiming mechanics you can find in an FPS. Its author, Nooky, kindly allowed me to translate this article in English and post it here. All kudos (and potential complaints ;)) goes to him. Unfortunately, I can't really translate the embedded videos, but everything is explained in the following paragraphs.

Original article.