line-spacing

line-spacing is a variable defined in `C source code'.
Its value is

nil

• Automatically becomes buffer-local when set.

Documentation:
Additional space to put between lines when displaying a buffer.
The space is measured in pixels, and put below lines on graphic displays,
see `display-graphic-p'.
If value is a floating point number, it specifies the spacing relative
to the default frame line height. A value of nil means add no extra space.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.