The term apostle comes from the Greek Απόστολος, which means sent. An apostle is a propagator or preacher of the biblical doctrine of the Christian faith and the power and the love of God, is an evangelizer whose mission is to preach Jesus Christ and His work Redentora, His life, His death and His Resurrection.
Each of the twelve main disciples chosen by Jesus Christ to preach and spread the Gospel (in Greek, good news) throughout the world are called Apostles (capitalized).
According to the gospels, which are books written by some disciples of Jesus about His life, these were the Apostles appointed by Jesus Christ: Simon Peter, James the Greater, Andrew, John, Philip of Bethsaida, Bartholomew, Thomas, Matthew, James the Lesser, Judas Tadeo, Simon and Judas Iscariot, the latter was replaced by Matthias after the suicide of Judas and the Ascension of Jesus.
Paul of Tarsus and Barnabas were also Apostles although they were not directly called by Jesus Christ when He was on Earth, but later. They all had the mission of expanding the Kingdom of God, and in addition to preaching the Gospel, they performed different wonders, signs and miracles, such as healing the sick, resurrecting, casting out demons, etc.
Some religions or Christian denominations believe that there are apostles today, for example the so-called New Apostolic Church, or the Church of Jesus Christ of Latter-day Saints, which has a Quorum of the Twelve Apostles that are present today.
In general, any person who spreads a political and social idea, or preaches a doctrine or religious belief is called an apostle.