A dude is an individual, typically male. The female equivalent, which is used less often, is "dudette" or "dudess". However, "dude" has evolved to become more unisex to encompass all genders, and this was true even in the 1950s.
The word dude is an American English slang term generally used informally to address or refer to somebody and was once used primarily by adults but this has become a common slang term used in various age groups.
The word became prominent in surfer culture in the early 1960s, but it wasn't until the mid-'70s that it started creeping into the mainstream. Some usages in mainly American pop culture have contributed to the spread of this word.
Guys use the word to refer to other guy friends as "Dude" when talking to them.