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 `水` 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 `水` 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