To begin, the definition of "disciple", according to Merriam-Webster is one who accepts and assists in spreading the doctrines of another: such as Christianity: one of the 12 in the inner circle of Christ's followers according to the Gospel accounts, or a convinced adherent of a school or individual (i.e. disciple of Freud).
Dictionary.com defines it as this: (1) Religion (one of the 12 personal followers of Christ; one of the 70 followers sent forth by Christ; any other professed follower of Christ in His lifetime); (2) any follower of Christ; (3) a member of the Disciples of Christ; (4) a person who is a pupil or an adherent of the doctrines of another; follower.
So pretty much, these two definitions seem to match. And what's more, they both use a couple of words that are synonyms of "disciple:" adherent, follower. Other synonyms include: student, pupil, teacher, mentor, influence, learn.
But what does it mean to BE a disciple? Well, from what I understand of the Bible and teachings of Jesus it means to be a student of Him and His teachings. To follow Him and live out His teachings in our daily lives. To teach others about Him. To spread the Good News! Jesus even commanded us, his followers, to share Him with others in Matthew 28:19.