in 802.1x, an entity at one end of a point-to-point LAN segment that facilitates authentication of the entity attached to the other end of that link.
An authenticator is something which determines authenticity or which guarantees validity. An authenticator is usually an object, a piece of knowledge, or some characteristic of it's possessor. It is typically uniquely in the possession of an entity so that the entity can prove it's authenticity (in an interaction) by demonstrating that it has possession of the authenticator.
A symbol or group of symbols, or a series of bits, selected or determined in a prearranged manner and usually inserted at a prearranged point in a message or transmission in order to determine the validity of the message or transmission or the identity or eligibility of an individual or entity. Cookies are often used as authenticators in client authentication schemes on the web.