What is the meaning of Ascendancy?

The quality of being in the ascendant; dominant control, supremacy.

Ellipsis of Protestant Ascendancy, a class of Protestant landowners and professionals that dominated political and social life in Ireland up to the early 20th century.

A quantitative attribute of an ecosystem, defined as a function of the ecosystem's trophic network, and intended to indicate its ability to prevail against disturbance by virtue of its combined organization and size.

Source: wiktionary.org