The Collaborative International Dictionary of English v.0.48:

Selector \Se*lect"or\, n. [L.]
   One who selects.
   [1913 Webster]

WordNet (r) 3.0 (2006):

selector
    n 1: a person who chooses or selects out [syn: picker,
         chooser, selector]
    2: a switch that is used to select among alternatives [syn:
       selector, selector switch]

The Free On-line Dictionary of Computing (30 December 2018):

selector

    1. In Smalltalk or Objective C, the syntax
   of a message which selects a particular method in the target
   object.

   2. An operation that returns the state of an object but does
   not alter that state.  Selector functions or methods often
   have names which begin with "get" and corresponding modifier
   methods or procedures whose names begin with "set".

   (1998-01-12)