ility

ility is a productive noun-forming suffix in English used to express a quality, state, or degree associated with the base word. It attaches to adjectives (and sometimes other stems) to create abstract nouns such as fragility, stability, mobility, viability, feasibility, visibility, and utility. The -ility ending comes from Latin -ilitas and entered English via Old French -ilité, becoming one of the common ways to derive nouns from adjectives.

In usage, -ility nouns often denote an inherent characteristic or a capacity or potential under certain conditions. They are common in scientific, philosophical, legal, and technical writing, where precise descriptions of properties and conditions are needed. Many -ility words originate from adjectives ending in -ible or -able (for example, feasible → feasibility, visible → visibility) or from adjectives with a root related to -il- (fragile → fragility, stable → stability). Nevertheless, there are exceptions, and not all -ility nouns follow a single uniform formation rule.

Phonology and spelling typically render the suffix as -ility, with the final pronunciation often /-ɪlɪti/ and the main stress placed on the syllable before the -ity ending in everyday usage. Because English morphology blends many sources, some words ending in -ility may also be analyzed as containing an -ibility ending (as in visibility, feasibility, or responsibility), reflecting the Latin origin of the suffix family.

See also: -ity, -ible, -able, -ibility, -ability.