There are 44 CSS length units

Adam Argyle has written a Codepen that lists the different units.

To learn more about what each unit means:

Relative

Unit Definition
% Percentage of the parent element's font size
em Font size of the element
ex Height of the element's font
ch Width of the "0" (zero) character of the element's font
cap Hight of the capital letters of the element's font
ic Advance measure of the CJK (Chinese/Japanese/Korean) character of the element's font
lh Height of the element's line height

Root Relative

Unit Definition
rem Font size of the root element
rex Height of the root element's font
rch Width of the "0" (zero) character of the root element's font
rlh Height of the root element's line height
ric Advance measure of the CJK (Chinese/Japanese/Korean) character of the root element's font
rcap Height of the capital letters of the root element's font

Absolute

Unit Definition
px Pixels
pt Points
pc Picas
in Inches
cm Centimeters
mm Millimeters
Q Quarter-millimeters

Viewport

Unit Definition
vw Viewport width
vh Viewport height
vi Viewport inches
vb Viewport breadths
dvw Dynamic viewport width
dvh Dynamic viewport height
dvi Dynamic viewport inches
dvb Dynamic viewport breadths
svw Smallest possible viewport width
svh Smallest possible viewport height
svi Smallest possible viewport inches
svb Smallest possible viewport breadths
lvw Largest possible viewport width
lvh Largest possible viewport height
lvi Largest possible viewport inches
lvb Largest possible viewport breadths
vmin Smallest value between the viewport's width and height
vmax Largest value between the viewport's width and height

Container

See CSS Container Queries.

Unit Definition
cqw Container width
cqh Container height
cqi Container inches
cqb Container breadths
cqmin Smallest value between the element's parent container's width and height
cqmax Largests value between the element's parent container's width and height