The English term “apostle” is derived from the Greek word “apostello”, meaning “to send forth.” It is hardly used in the Old Testament, but appeared at least 80 times in the New Testament. Based on Catholic Encyclopedia, Jesus may have originally used the Aramaic word “seliah”, which describes “those who were dispatched from the mother city by the rulers of the race on any foreign mission, especially such as were charged with collecting the tribute paid to the temple service.”
Therefore, an apostle is someone who is “sent” by our Lord to the nations. For several centuries, this was almost exclusively used in addressing the 12 apostles, who were sent by Jesus into the world.
It is also used for great saints who were sent by the Lord on a particular mission. An example of this is Saint Boniface, known as the “Apostle to the Germans,” for his missionary work among the Germanic people.
Meanwhile, the English word “disciple” is obtained from the Latin “discipulus”, meaning a “student” or “pupil”. It is also used almost exclusively in the New Testament. It usually denotes the several “students” who surrounded Jesus and learned from His teachings. Given this context, a disciple is not necessarily a person who is “sent” to preach the God’s Word to the world, but someone who constantly learns what it means to be a Christian.