The act of putting all availble force into one attack.
1) To fire the whole arsenal at one target
2) To Launch all fighter planes against one military target.
3) To drop all availble bombs.
4) To deplete all potential firepower at once.
5) To fire a broadside.
A large and generally overwhelming attack meant to disable or destroy an opponent quickly. Generally speaking it is the result of an overuse of large and powerful weaponry with a long refire in order to minimize penalties associated with said weapon systems.
Originally the term was used in the game Mechwarrior, but it has since migrated to become common usage in other mediums and genres.
The alpha strike capability on that thing is very powerful!