A more masculine woman who is a Lesbian, who is considered to be more 'like a man' than some men.
Often used in a derogatory sense, it refers to a Lesbian with more manly attributes, such as their clothing style, their mannerisms, and their attire. In general it referred to woman as being masculine, devoid of feminine charm or grace.
It has been reclaimed, to mean assertive, tough, but not mean spirited. More of a word of 'pride' by some, while other's still consider it a slur.
It's basic meaning was to refer to a Lesbian. In many circles today, it's only meaning is that, without any stigma or other attributes given to its meaning or connotation.
There is no definitive history of how the word became attached to Lesbians, though at the beginning it was not said in a complimentary sense.
Bulldyker is considered to be the originating term, which might have been an abbreviation for morphadike, a variant on 'hermaphrodite' which was the term referencing homosexuals in the 19th century.
Another possible origin relates to the term which mean a 'bull' that serviced 'cows' only, and it might have been co-opted to include masculine looking or acting woman who serviced other woman, rather than a male.
History (Legend): First referenced in print, in a book during the Harlem Renaissance time, which suggested the word had an original term of 'bulldyker' which was shortened to just 'dyke'.
Considered to be woman who are assertive, dress similar to their male counter parts, and like a male, are the more aggressive in sex. Usually a 'dyke' is the 'top' in a relationship.
Noteworthy: Traditional Gay Pride Parades are led off by women bikers, who termed themselves Dykes On Bikes. An attempt to 'trademark' the term was rejected initially due to the consideration that the word 'dyke' was offensive. It was only after a lengthy court battle was the 'trademark' granted.