An Etymological Dictionary of Astronomy and Astrophysics

1. The act or an instance of expressing.
   

2. Math.: A statement using mathematical quantities such as → scalars, → variables, → parameters, → functions, and → sets, as well as → relational and → logical  → operators such as → equality,
   → conjunction, → existence, → union, etc. (Steven. G. Krantz, ed., Dictionary of Algebra, Arithmetic, and Trigonometry, 2001).

See also: → express; → -tion.