Beta
Also said as Content complete, Open beta, Closed beta
The stage when all features and content are in and the work is finishing and fixing. A public beta opens this to outside players, usually to test servers and balance.
You find out how a game stands up to a crowd only by having one. A beta weekend gets that answer at a time when failure costs little.
Say it
“Run a closed beta with 5,000 players in May, then an open weekend in June, with server capacity set at half our launch estimate so we hit the limit on purpose.”
Where you’ve seen it
Diablo III ran an open beta weekend in April 2012, and on launch day that May still met a login rush large enough that "Error 37" became a joke among players. A beta tests the size of crowd it gets.
Not to be confused with demo
A demo is a finished sample made to sell the game. A public beta is unfinished, and is there to test the game. Plenty of public betas are also marketing, and players can tell.