I'm sure this is not a new insight, but I just realized that μαθητής is only found in the Gospels and Acts; and not in any other NT book (at least, not the NA/UBS text). It also isn't found in the LXX. It does occur a few times in the Ignatian epistles, three times in the Martyrdom of Polycarp, and a few times in the Epistle to Diognetus (which purports to be from "μαθητής", so it is to be expected there, isn't it?).

The Exegetical Dictionary of the New Testament (EDNT) has the following snippet, among other items:

The noun μαθητής appears 261 times in the NT. It is found only in the Gospels and Acts (Matthew has 72 occurrences, Mark has 46, Luke has 37, John has 78, and Acts has 28). The vb. μαθητεύω appears 4 times (Mt 13.52; Mt 27.57; Mt 28.19; Acts 14.21).

Balz, H. R., & Schneider, G. (1990-c1993). Exegetical dictionary of the New Testament. Translation of: Exegetisches Wörterbuch zum Neuen Testament. (vol. 2, p. 372). Grand Rapids, Mich.: Eerdmans.

I've poked through a few articles (EDNT as well as glimpsing at TDNT) but haven't found a nice, short summary or even a reference to a book dealing specifically with the subject. I can't look into it anytime soon, but it just seems weird to me that the term just stops being used. After all, Jesus said to "make disciples" in his name (Mt 28.19) — why don't the epistles report this using that term?

Thoughts/comments/references/whatever appreciated; feel free to use comments or to send email my way using the address on the sidebar.

Thanks!