Line height
The distance from one line of text down to the next, baseline to baseline.
See it in Text flow. Move Line height and watch the lines spread apart and close up.
In plain words
Line height is how far apart the lines of a paragraph sit. It is often given as a multiple of the font size, such as 1.5. Too tight, and tall letters from one line crowd the line above. Too loose, and the eye loses its place moving to the next line. Reading text is usually set around 1.4 to 1.6.