#dimensionless unit

s⁰⋅m⁰⋅kg⁰⋅A⁰⋅K⁰⋅mol⁰⋅cd⁰

also known as:

a dimensionless quantity

this makes this a really weird unit to start on; the next few will be much more normal, but this is the definite "first" one in the sequence

radian (rad)

measure of angle

units of m/m = 1

commensurable with: degree/minute/second, grad, turn/revolution, milliradian, binary degree (i like this one because the abbreviation is BAM which is pretty funny)

steradian (sr) (m²/m²)

measure of solid angle

commensurable with: square degree, spat

there are also many non-SI defined dimensionless quantities, such as:

the lorentz factor

mach number

atomic weight

pH

friction

decibel