An "alpha strike" is when a BattleMech attacks with all of its weapons at the same time. While extremely powerful, it dramatically raises the 'Mech's heat level, and after several successive alpha strikes most 'Mechs will shut down from overheating.
 Game play
In the BattleTech CCG, an Alpha Strike was referred to in the manual as an attack where the unit completely exhausted all its autocannon ammo. This increased the unit's attack, but caused it to be depleted afterward.
In MechWarrior: Dark Age and Age of Destruction the effect is far greater: while increasing the damage value of its attack by the number of clicks the 'Mech has left until shutdown, the 'Mech shutdown is automatic, exposing it to powerful counter-attacks.