A ring $A$ with one or more distinguished derivations (cf. Derivation in a ring). An element $a \in A$ such that $d(a) = 0$ for all these derivations $d$ is said to be a constant. The constants form a subring of $A$.
A differential field is a differential ring that is a field. The set of constants of a differential field is a subfield, the so-called field of constants.
Differential field. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Differential_field&oldid=39573