Dictionary human definition:
the abandonment or renunciation of a religious or political belief.
The definition according to Gods word:
1Tim 4:1 But the Spirit saith expressly, that in later times some shall fall away (αποστησονται = apostEsontai = shall-be-withdrawing)* from the faith, giving heed to seducing spirits and doctrines of demons
Luke 8:13 Those on the rock are the people who on hearing the Message receive it joyfully; but they have no root: for a time they believe, but when trial comes they fall away (αφιστανται = aphistantai = they-are-withdrawing)* – Weymouth
2Thess 2:3-4 Let no one in any way deceive you, for that day cannot come without the coming of the apostasy (αποστασια = apostasia)* first, and the appearing of the man of sin, the son of perdition, who sets himself against and exalts himself above, every so-called 'god' or object of worship, and goes the length of taking his seat in the very temple of God, giving it out that he himself is God. – Weymouth
From the above verses it can be concluded that according to scripture an apostate is one who turns away from belief in God, Christ, and Gods Word of Truth after having found it. The view of course of the “Christian” world religion is that if you leave their particular sect or belief then you are considered an apostate by their standards even if you have not withdrawn from God, Christ or the truth of God's Word in your actions and belief.
*added for clarity