Definitions
1.NounOne who calls upon (a person, especially a god) for help, assistance or guidance."An invoker of God's mercy"
2.NounOne who appeals for validation to a (notably cited) authority.
3.NounOne who conjures up spirits with incantations.
4.NounSomeone who induces as an inevitable consequence.
5.NounOne who solicits, petitions for, appeals to a favorable attitude.
6.NounThat which causes a program or subroutine to execute.