I want to define straight using the definition of having 180 degrees, but how do you explain 180 degrees?
Also what types of symmetry are considered line symmetry?
First a comment. Many who have worked in axiomatic geometry do not like using ‘degrees’ as a measure of angles.
An angle is the union of two rays with a common endpoint; as in $\displaystyle
\angle BAC = \overrightarrow {AB} \cup \overrightarrow {AC} $
Now a straight angle would be the case in which $\displaystyle C-A-B$.
$\displaystyle A,~B,~\&~C$ are collinear and $\displaystyle A$ is between $\displaystyle B~\&~C$.