An Etymological Dictionary of Astronomy and Astrophysics

1. That part of the mouth or lower course of a river in which the river’s current meets the sea’s tide.
   

2. An arm or inlet of the sea at the lower end of a river (Dictionary.com).

Etymology (EN): From L. aestuarium “a tidal marsh, mud-beds covered by water at high tides; channel inland from the sea,” from aestus “boiling (of the sea), tide, heat,” from PIE *aidh- “to burn;” cognates: Gk aithein “to burn,” Skt. inddhe “burst into flames,” O.Irish aed “fire,” O.H.G. eit “funeral pile.”

Etymology (PE): Kešandân, literally “site of tides,” from kešand, → tide,

• -ân a suffix of place and time.