1. From Japanese Ha Dou (Wave or Surge) and Ken (Fist, Technique, or Attack); a blastwave or fireball formed from a martial artist's life energy (or ki/ch'i). The hands are brought together, heels of the palms touching, and moved to beside the fighter's waist (which is as close as one can get to the center of the body, where the most ch'i gathers). The energy is then focused into the hands, and when both arms are thrown forward, palms outward, the energy wave is fired in the same direction as the double palm punch. Very similar in nature to other ch'i-based fireball attacks, including Son Goku's _kamehameha_, Piccolo's _ma kou sen_, the fireballs used by Liu Kang and Johnny Cage, and other cast members of Mortal Kombat, Fatal Fury, and especially Street Fighter. The Hadoken technique itself was developed by the shotokan karate master Gotetsu, who taught it to his disciple Goken, who in turn taught it to Ryu and Ken Masters, along with his other special techniques, the ShoRyuKen (Dragon Power Fist) and the TatsuMakiSenPuKyaku (Hurricane Kick).
2. In the webcomic _8-Bit Theater_, by Brian Clevinger, the character Black Mage casts a 9th-level spell called Hadoken. The spell seems to be a pumped-up version of the Final Fantasy I spell "NUKE" (also called "FLARE" in later sequels of the game), though in Final Fantasy I, there were only 8 spell levels (9 being the number of spell levels traditionally used in Dungeons & Dragons).
You do not yet have enough power to produce the Hadouken.