LSJ
τό, middle of navel, Poll. 2.169; — also ἀκρόμφαλον, τό, Ruf. Onom. 98.
Liddell-Scott-Jones, Greek-English Lexicon (9th ed., 1940)
Pape
τό, Poll., = ἀκρόμφαλον, τό, Sp., Mitte des Nabels.
Pape, Griechisch-deutsches Handwörterbuch (3. Aufl., 1914)