Triangles ABC and A'B'C' are similar.
Given: m' > m.
How to pronounce ' (штрих in Russian)?
The word that you're looking for is: prime.
"The prime symbol ( ′ ), double prime symbol ( ″ ), triple prime symbol ( ‴ ), quadruple prime symbol ( ⁗ ) etc., are used to designate units and for other purposes in mathematics, the sciences, linguistics and music."
See also here, for more examples how to use this word in sentences.