The term MILF stands for Mother I'd Like to Fuck.
However it's not normally associated with the escort business, but refers to an attractive older woman with grown up kids, who has become the focus of attention of a young single male.
The term became popular as a result of the movie American Pie, and in The Graduate film a young Dustin Hoffman has an affair with the mother (MILF) of a college girl that he was seeing at the same time.