In physics, defining equations are equations that define new quantities in terms of base quantities. 7 Fundamental Quantities

symbol	meaning	units	SI unit	basic SI units
l	Length	meter	`m`	`m`
m	Mass	Kilogram	`kg`	`kg`
t	Time	second	`s`	`s`
I	Electric current	ampere	`A`	`A`
T	Temperature	Kelvin	`K`	`K`
n	Amount of substance	Mole	`mol`	`mol`
lv	Luminous intensity	candela	`cd`	`cd`

For Example, Velocity:

symbol	meaning	units	SI unit	basic SI units
t	Time	second	`s`	`s`
v	Velocity	meter per second	`m//s`	`m//s`
∆x	displacement	meter	`m`	`m`

Newton's 2nd low:

symbol	meaning	units	SI unit	basic SI units
m	Mass	Kilogram	`kg`	`kg`
F	Force	newton	`N`	`kg•m//s^2`, (`J//m`)
`a`	acceleration	meter per second square	`m//s^2`	`m//s^2`

Displacement under constant acceleration:

symbol	meaning	units	SI unit	basic SI units
t	Time	second	`s`	`s`
v	Velocity	meter per second	`m//s`	`m//s`
`a`	acceleration	meter per second square	`m//s^2`	`m//s^2`
∆x	displacement	meter	`m`	`m`