The headword, feminine aorist participle of de/xomai, has here the specific meaning of the baptistery in Hagia Sophia (the place for receiving converts) -- although the lexicographer interprets it differently.
[1] Procopius, Secret History 3.24-5 (web address 1), abridged. In the latter part, 'her' (au)th/n) is a slip here for 'him' (au)to/n), i.e. Theodora's enemy Photius (cf. alpha 1492 note). Photius had escaped from Theodora's prison and sought refuge at Hagia Sophia; cf. Kaldellis (16-17). On Theodora, see alpha 956 note.