What does the abbreviation U.FL-LP connector stand for?
U.FL-LP is just the series-name of a particular kind of connector, made by Hirose.

Despite Hirose having patents on those connectors, U.FL compatible connectors can be found from other manufacturers (https://en.wikipedia.org/wiki/Hirose_U.FL).

In the datasheet, there is no indication as to what U.FL stands for. One might try to guess that U stands for micro (as in the μ symbol), LP for low profile, but there is no official indication from Hirose.