Equi

Equi is a combining form used in English to mean equal or even. It derives from the Latin aequus and is used chiefly in technical terms to indicate that a property applies equally across multiple components or positions. The form equi- typically attaches to a root starting with a vowel, or to create a smooth, phonetic word when the meaning concerns uniformity or balance.

In geometry and related fields, equi- yields terms such as equilateral (all sides equal) and equiangular (all angles equal). In measurements and chemistry, equi- appears in equidistant (the same distance from a reference point) and equimolar (equal moles of substances). In physics and engineering, equipotential describes surfaces or lines of equal electric potential, while equilibrium denotes a state of balance between opposing forces or processes. The prefix also appears in historical terms like equinox, referring to times when day and night are of equal length.

Usage notes include that equi- is distinct from the standalone word equal in everyday language; equi- is primarily a productive prefix in compound terms. It can be contrasted with related Latin-derived roots such as equi- and equ- found in equality, equivalent, or equity, which share a common ancestor but are not all formed with the equi- prefix.

A caution: equi- should not be conflated with equine, which refers to horses and derives from a different Latin root (equus). Equi- and equine are etymologically distinct despite similar spelling.