noun

• The quality of being in the ascendant; dominant control, supremacy.
• (ecology) 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.
• (historical, Ireland, sometimes capitalized) 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.