Mega Man X
The Hadouken (波動拳 "Surge Fist") is a move from Capcom's Street Fighter series that was included in as a secret power-up obtained from a Light Capsule in Mega Man X, Maverick Hunter X, Mega Man Xtreme and Mega Man Xtreme 2, and is executed in the same way, inputting the combination ↓↘→+ATTACK while X's health gauge is full and he is on the ground to unleash a powerful energy ball. It can kill most bosses and enemies in a single hit.
In Mega Man X4 Hadouken is one of Magma Dragoon's attacks. In Mega Man X: Command Mission the boss Ninetails uses the Annihilator Hadoken (滅殺波動拳 Messatsu Hadouken). In Project X Zone X uses a powerful Hadouken alongside Zero's Genmurei.
Street Fighter X Mega Man
Hadouken is Ryu's special weapon from Street Fighter X Mega Man. When used, Mega Man will release a small and slow Hadouken, and can be charged to shoot a quick yellow Hadouken. The Hadouken is the weakness of Chun-Li, M. Bison, and Akuma.
The Charged Hadouken does extra damage to akuma and M.Bison, However it does regular damage to Chun-Li, Thus when fighting Chun-Li you can spam regular hadoukens with the same affect.