Only here in New Testament. Better, as Rev., fellow-elder. The expression is decisive against the primacy of Peter.
Witness (μάρτυς)
The word is used in the New Testament to denote (a) a spectator or eye-witness (Act 10:39; Act 6:13). (b) One who testifies to what he has seen (Act 1:8; Act 5:32). (c) In the forensic sense, a witness in court (Mat 26:65; Mar 14:63). (d) One who vindicates his testimony by suffering: a martyr (Act 22:20; Heb 12:1; Rev 2:13; Rev 17:6). The first three meanings run into each other. The eye-witness, as a spectator, is always such with a view to giving testimony. Hence this expression of Peter cannot be limited to the mere fact of his having seen what he preached; especially since, when he wishes to emphasize this fact, he employs another word, ἐπόπτης (2Pe 1:16). Therefore he speaks of himself as a witness, especially in the sense of being called to testify of what he has seen.
Partaker (κοινωνός)
This use of the word, expressing a present realization of something not yet attained, occurs in no other writer in the New Testament. See on 2Pe 1:4.