Derek is a male name. It is the English language short form of Dederik, the Low German form of the name Theoderic. Theoderic is an old Germanic name with an original meaning of "people-ruler".
Common variants of the name are Derrek, Derick and Derrick, more rarely also Deryck or 'Derrok'. The Dutch language short form of the same Low German / Low Franconian name is Dirk (popularised by the actor Dirk Bogarde) and the Low German Di(e)rk (Dierk Hoff).
Read more about Derek: History, Use As A Surname, Fictional Characters