"HAIL HYDRA! Cut off a limb and two more shall take its place!"
High-tech global criminal organization in Marvel Comics that frequently opposes Captain America, S.H.I.E.L.D., and the Avengers.
HYDRA was formed during World War II by Baron Von Strucker as a means of extending the Red Skull's organization into the far East. After Germany's defeat HYDRA continued to work toward global domination and the triumph of fascism until Nick Fury and S.H.I.E.L.D. destroyed their island headquarters, whereupon the organization broke apart into several splinter groups.
HYDRA eventually re-formed with an emphasis on terrorism and crime. Silvermane and Viper (as "Madame Hydra", stunning in dark green tights, green-tinted hair, and carrying a whip) have both held the position of Supreme Hydra; Jessica Drew (Spider-Woman) began her career as a HYDRA assassin. Eventually Baron Von Strucker came back and imposed his Nazi ideology on the group once more.
Unlike real terrorists, HYDRA agents are easily spotted in their green uniforms and red-tinted goggles, and are armed with extraterrestrial technology obtained by the group near the end of the war. Occasionally they will kidnap someone and replace him or her with a robotic duplicate known as a Life Model Decoy*. AIM acted as their scientific branch until its leadership was seized by MODOK, who severed AIM's ties with HYDRA.
*My knowledge of this bit of trivia helped settle a debate between Howard Chaykin and Frank Miller once when they were signing at my local comics store. HAIL NAME-DROPPING!