A horizontal line placed under a symbol to indicate some special property. Underscores are sometimes used instead of over-arrows or bold typeface to indicate a vector,
for example 
.
Underscore
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References
Bringhurst, R. The Elements of Typographic Style, 2nd ed. Point Roberts, WA: Hartley and Marks, p. 286, 1997.Referenced on Wolfram|Alpha
UnderscoreCite this as:
Weisstein, Eric W. "Underscore." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Underscore.html