\begin{definition}[Definition:Equidistance]
'''Equidistance''' is one of the undefined terms in Tarski's Geometry.
Intuitively, two points $a, b$ are equidistant with respect to $c, d$ if the length of line segment $a b$ is the same as that of line segment $c d$. 
However, at this point in the game, we have not yet defined line segment, or even distance.
As such, let the following ostensive definition suffice.
File:Equidistance.png
In the diagram, points $a, b$ are '''equidistant''' to points $c, d$, and we write $a b \equiv c d$.
However, points $a, d$ are ''not'' '''equidistant''' to points $a, b$, and we write $\neg \paren {a d \equiv a b}$ or $a d \not \equiv a b$.
\end{definition}