Apostle does come from the Greek word apostolos
/ apostello
, as noted by Mason WheelerMason Wheeler, and it does indicate one who is sent out.
In the Gospels, the Twelve are most commonly referred to as disciples. They were following the Teacher, learning from Him as His students.
In John 17:18 and Matthew 28:18-20, among other verses, Jesus specifically sends them out and commissions them with the Great Commission.
So, those who were once followers of Jesus became those who were no longer bid to come and follow, but also to go out and proclaim a message.
Even in Matthew 4:19, the pattern for this was established. Jesus said, "Come follow me [be disciples], and I will make you fishers of men [apostles].
This is also the call of everyone who comes to Jesus. We must first and foremost follow Him, but we are also sent out with the Great Commission. Every disciple is called to be an apostle - a sent one.