Someone who has only Free Software installed on their boxen, for example. Or someone who will only play acoustic music, or some specific style(s) of music. You get the idea...

Can't live with 'em, can't live without 'em.

Pur"ist, n. [Cf. F. puriste.]

1.

One who aims at excessive purity or nicety, esp. in the choice of language.

He [Fox] . . . purified vocabulary with a scrupulosity unknown to any purist. Macaulay.

2.

One who maintains that the New Testament was written in pure Greek.

M. Stuart.

 

© Webster 1913.