An expression that is of a given type. For example, all primes 
are "of the form" 
. The term "of shape" is sometimes also used.
Of the Form
See also
Represented AsExplore with Wolfram|Alpha
References
Wells, D. The Penguin Dictionary of Curious and Interesting Numbers. Middlesex, England: Penguin Books, p. 13, 1986.Referenced on Wolfram|Alpha
Of the FormCite this as:
Weisstein, Eric W. "Of the Form." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/OftheForm.html