Wiktionary
uterine isthmus
n. (context anatomy English) An elongated, narrow part of the uterus at the junction of the body of the uterus and the cervix.
Wikipedia
Uterine isthmus
The uterine isthmus is the inferior-posterior part of uterus, on its cervical end — here the uterine muscle ( myometrium) is narrower and thinner. It connects superiorly-anteriorly to the complementary parts of the uterus: the body and the fundus.
The uterine isthmus can become more compressibile in pregnancy, which is a finding known as Hegar's sign.