| HYDRA is a fictional terrorist organization in the Marvel Universe. Despite the name's capitalization per Marvel's official spelling, the name is not an acronym but rather a reference to the mythical Lernaean Hydra. The organization's motto references the myth of the Hydra, stating that "if a head is cut off, two more will take its place" proclaiming their resilience and growing strength in the face of resistance. HYDRA agents often wear distinctive green garb featuring a serpent motif.
HYDRA was first mentioned in Menace #10. In that issue, a plainclothes HYDRA agent paid off a scientist named Dr. Nostrum for information about a cobalt bomb that turned people into monsters. Dr. Nostrum shot all the other scientists on his team after they were turned into monsters, then shot himself after his son put an image from a monster magazine on his mirror.
The organization first appeared in Strange Tales #135. In its original continuity, it was headed by nondescript businessman Arnold Brown, who was killed as S.H.I.E.L.D. apparently crushed the organization. It soon returned, however, headed by Baron Wolfgang von Strucker, with the support of the Nazi Red Skull; HYDRA's changing origin was one of Marvel's earliest retcons. After its initial defeat, several of its branches, such as its scientific branch A.I.M. (Advanced Idea Mechanics) and the Secret Empire, became independent.
Source: Wikipedia, "HYDRA", available under the CC-BY-SA License.