\begin{definition}[Definition:Between (Geometry)]
'''Betweenness''' is one of the undefined terms in Tarski's Geometry.
Intuitively, a point $b$ is between two others $a$ and $c$ {{iff}} it lies on the line segment $a c$.
However as ''line segment'' has not yet been defined, we are not allowed to call upon it at this stage.
We offer an ostensive definition:
:File:Betweenness.png
In the picture, point $b$ is '''between''' the two points $a, c$, and we write:
:$\mathsf{B}abc$
However, point $d$ is ''not'' '''between''' the two points $a$ and $c$, and we write:
:$\map \neg {\mathsf B a d c}$
\end{definition}