Independent (Indie) Developer
An indie developer is a developer - singular or a team - of a game - regardless of quality or system - which is created without the aid of a larger AAA studio. Examples of indie developers include: Team Meat (Super Meat Boy); Gears for Breakfast (A Hat in Time); thatgamecompany (flOw, Journey).
Intellectual Property (IP)
An IP is a franchise - including single-entry franchises. This doesn't just extend to games, but all forms of media and art - film and TV, plays, books, etc.
Technology
Digital distribution has made it easier for smaller independent studios to get their work to the masses. Example digital storefronts for smaller games: Xbox Live Arcade (defunct); itch.io; GameJolt; NewGrounds.
Laws and Regulations
While the indie developers have made the game, they do not own the IP - the publisher or sponsor does. Freelance artists also do not own their work despite the fact they themselves made it.
Industry Structure
Titles that draw more revenue (Madden NFL, Halo) require levels of technical development that make independent funding and development of original ideas near impossible.
Organisational Structure
Mobile and Internet games can be completed with one or two developers, but the possibilities for this structure are becoming more implausible.
Occupational Careers
People have left their full-time game dev jobs for indie development for a variety of reasons, namely: desires for more real and autonomous creation; a feeling of not being able to move up in the hierarchy; a discontent about management.
Market
Duplicates are more recognisable, giving the idea that indie developers are becoming more like the larger companies they've escaped from.
Mod communities have grown as a result of the blending of producers and consumers in indie games.