In a right angle triangle if the lengths of the adjacent or the hypotenuse are known.
a right angle is a right angle
It has to be 90 degrees and yes, straight.
The term of Latin in right angle is angulus rectus; here rectus means "upright", referring to the vertical perpendicular to a horizontal base line.
"Right" has other meanings, apart from the opposite of "left". I suspect that the ethymology here is more like a "correct" angle.