Whatever name you have given the unit. Open the unit propeties for the unit you want to check against (double click it), there you'll find a box to put in a "name", give it a name "general".
Then in a trigger, put in the condition;
!alive General
You'll find that alot of people on the forums will replace words in the commands with generic terms. Like UnitName or things like that. Normally, if they are nice they will put the parts of the code you can change in italics, or colour, or some other type of formatting to make things obvious.