The betting structure where there is not a limit on the maximum bet up to the value of chips in front of him.
A betting structure in which there is no maximum bet. Players may bet as much as they want at any time, at any time they can declare themselves "all in" and put all of their chips into the pot.
A version of poker in which a player may bet any amount of chips (up to the number in front of him) whenever it is his turn to act. It is a very different game than limit poker. The best treatise on no-limit poker is in Doyle Brunson's Super/System.