Definition:Relative Complement

Definition
Let $S$ be a set, and let $T \subseteq S$, that is: let $T$ be a subset of $S$.

Then the set difference $S \setminus T$ can be written $\relcomp S T$, and is called the relative complement of $T$ in $S$, or the complement of $T$ relative to $S$.

Thus:
 * $\relcomp S T = \set {x \in S : x \notin T}$

Also known as
Some authors call this the complement and use the term relative complement for the set difference $S \setminus T$ when the stipulation $T \subseteq S$ is not required.

Others emphasize the connection with set difference by referring to the relative complement as a proper difference.

Thus, in this view, the relative complement is a specific case of a set difference.

Different notations for $\relcomp S T$ mainly consist of variants of the $\complement$:
 * $\map {\mathcal C_S} T$
 * $\map {\mathbf C_S} T$
 * $\map {c_S} T$
 * $\map {C_S} T$
 * $\operatorname C_S \paren T$

or sometimes:
 * $\map {T\,^c} S$
 * $\map {T\,^\complement} S$

... and sometimes the brackets are omitted:
 * $C_S T$

If the superset $S$ is implicit, then it can be omitted: $\complement \paren T$ etc. See the notation for set complement.

Some sources do not bother to introduce a specific notation for the relative complement, and instead just use the various notation for set difference:
 * $S \setminus T$
 * $S / T$
 * $S - T$

Also see

 * Definition:Set Difference
 * Definition:Set Complement