An Etymological Dictionary of Astronomy and Astrophysics

A parameter designating the rate at which the expansion of the Universe would slow down owing to the braking gravitational effect of the matter content of the Universe. It is expressed by:
q(t) = -R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. See also
→ expansion parameter; compare with → acceleration parameter.

See also: → deceleration;
→ parameter.