The units in CSS may be defined as an absolute or a relative value; the list of which is written below:

Units
UNIT DESCRIPTION TYPE
% percentage relative value; percentage of the container block
in inch absolute value
cm centimeter absolute value
mm millimeter absolute value
em letter "M" (uppercase) relative value; in web design the length of 1em is equal to the current font size; i.e., if the font being used is currently set to 12px, then 1.5em will be 1.5 * 12 = 18px size font
ex letter "x" (lowercase) relative value; in web design the length of 1ex is the lowercase x-height of a current font; i.e. x-height is usually about half the font-size
pt point relative value; a point of an inch. In web design that 1pt is same 1/72 of an inch
pc pica relative value; in web design a 1pc is same as 12 points (12pt)
px pixels relative value; one pixel (1px) is a dot on the computer screen