Answer:
Generally speaking: A disciple is a "pupil"... a student... one who receives the teachings or instructions of another, believes in the teachings... and is active within its movement.
Although this applies to areas beyond "religion"... this question was categorically addressed to the use of "disciples" in the Bible.
Within the application of scripture, Jesus' definition of a disciple of His is:
"...If ye continue in My Word, THEN ARE YE MY DISCIPLES INDEED..." (John 8:31).
Jesus' instructions, teachings and doctrines, according to Him, are found in the Bible. He tells His disciples [those who would receive His intruction, believe it and commit to it] to "continue" in the Bible -- read it, study it... be edified by it.
"All scripture... is profitable [to Christ's disciples indeed] for doctrine, for reproof, for correction, for instruction in righteousness..." (II Tim.3:16).