ALGOL is a programming language that was first developed in the late 1950's; although little-used nowdays, it is historically very important as most moderm programming languages are developed from it, either directly, or indirectly (e.g. C, which is derived from BCPL, heavily influenced by ALGOL).
It introduced, among many other advances, modern control flow, including block structure. It was aptly described by C. A. R. Hoare, who worked on a later version of the language, as "a language so far ahead of its time, that it was not only an improvement on its predecessors, but also on nearly all its successors".
The first version, ALGOL 58, was defined by an international committee of people from different organizations (the first language to be created in this way). The first version to be widely implemented, ALGOL 60, differed from it in not too significant ways.
An attempted later revision, ALGOL 68, was very controversial (it was the inspiration for Hoare's famous Turing Award lecture, The Emperor's Old Clothes); it tried to greatly expand the language, and was not as successful as its predecessor.