An accent notation that places a horizontal line beneath text or expressions to indicate emphasis or special meaning.
Examples
Emphasizing a term in an algebraic expression
\underline{AB} + CD = EF
Highlighting a specific segment in geometry
\text{Length of segment } \underline{AB} = 5 \text{ units}
Marking a variable name in a physics equation
v_{\underline{AB}} = \frac{d\underline{AB}}{dt}