The Pin group \(\operatorname {Pin}(V ; q)\) of a quadratic vector space, is the subgroup of the group of units in the Clifford algebra \(\mathrm {Cl}(V, q)\)
\[
\operatorname {Pin}(V, q) \hookrightarrow \operatorname {GL}_1(\mathrm {Cl}(V, q))
\]
on those elements which are multiples \(v_1 \cdots v_n\) of elements \(v_i \in V\) with \(q\left (v_i\right )=1\).
The Spin group \(\operatorname {Spin}(V, q)\) is the further subgroup of \(\operatorname {Pin}(V ; q)\) on those elements which are even number multiples \(v_1 \cdots v_{2 k}\) of elements \(v_i \in V\) with \(q\left (v_i\right )=1\).