\begin{definition}[Definition:Fundamental Circuit (Matroid)]
Let $M = \struct {S, \mathscr I}$ be a matroid.
Let $B$ be a base of $M$.
Let $x \in S \setminus B$.
The '''fundamental circuit of $x$ in the base B''', denoted $\map C {x, B}$, is the unique circuit such that:
:$x \in \map C {x, B} \subseteq B \cup \set x$
\end{definition}