The Greek word apostolos means ‘a messenger,’ or one sent forth with orders. The New Testament speaks of three groups of apostles: Jesus’ 12 disciples, those who saw Jesus after His resurrection, and a few others. What did they all have in common? They were either witnesses to Jesus’ ministry, or to His resurrection, or they were sent by those witnesses to preach the gospel.
Those who preach and teach something other than what Jesus taught are called “pseudo-apostles” by Paul.