A diving-place, i.e. Pond for bathing (or swimming) -- pool.
Thayer's Greek Lexicon
Strong's 2861: κολυμβήθρα
κολυμβήθρα, κολυμβήθρας, ἡ (κολυμβάω), a place for diving, a swimming-pool (A. V. simply pool): John 9:7, and Rec. in 11; a reservoir or pool used for bathing, John 5:2, 4 ((according to text of R L), 7). (Plato, rep. 5, p. 453 d.; Diodorus, Joseph, others; the Sept., 2 Kings 18:17; Nehemiah 2:14; Nahum 2:8.)
John 5:2 - N-NFS GRK:τῇ προβατικῇ κολυμβήθρα ἡ ἐπιλεγομένη NAS: by the sheep [gate] a pool, which is called KJV: the sheep [market] a pool, which INT: the sheepgate a pool which [is] called