Unit:- For the correct measurement of all physical quantities,a certain and convenient quantity is considered as the standard. By comparing with this considered standard quantity,the magnitude of physical quantities are obtained. This considered standard quantity is called unit.
Properties of a unit: Any unit should have following properties.
(a) Its magnitude should be definite so that it can be obtained easily and accurately.
(b) Its magnitude does not change with time. That is ,atmospheric pressure and temperature should not affect it .
(c) It can be easily converted into other units.
Types of Unit:
Fundamental Units: Those units which are completely independent and have their own fundamental importance are called fundamental units.Example:- The units of Mass,Length,Time, Distance,displacement are fundamental units.
Derived Units: The units which are derived from the base units i.e. Which depend on the base units are called derived unites.Example:- unit of area(sq. cm), unit of volume(cc), unit of velocity(cm), Density, Force, weight, Pressure, Acceleration, Retardation these are all derived units.
Practical Units: The units which are mostly used in daily life are called Practical units,like unit of power(watt), unit of work(joule) these are practical units.
System of measurement: The systems which are used to measure and express physical quantities are called system of measurement. There are three main systems of measurement in the world.which are the Following.
(1) C.G.S System (2) F.P.S System (3) M.K.S System or S.I System

Units of some important physical quantities in different system
Quantities Symbol Unit in C.G.S system Unit in M.K.S(S.I)system Unit in F.P.S System
Area A Square centimetre (cm²) square metre (m²)
Distance or length l Centimeter(cm) Meter(m) Foot
Mass M Gram(g) Kilogram Pound
Velocity v Cm/sec Meter/sec ft
Acceleration a Cm/s² Meter/s² Ft/s²
Force F Dyne Newton(N) Poundal
Volume V Cubic centimeter(c.c) Cubic meter Cubic ft.(c.ft)
Time t Second(s) Second(s)
Electric Current i biot Ampere(A)
Temperature T Kelvin(K) Kelvin(K)
Speed s Cm/sec Meter/sec
amount of substance n Mole (mol)
luminous intensity Iv candela (cd)
Angular momentum L g-cm²/s kg-m²/s
Angular speed or angular velocity ω rad s−1 rad s−1
Capacitance C statfarad (statF) Farad (F)
wavenumber k reciprocal cm (cm-1) Reciprocal metre (m-1)
Current density J ampere/meter2(A/m2)
Electric charge Q statcoulomb (e.s.u) coulomb (C)
Electric potential V statVolt (statV) Volt (V)
Electrical resistance R ohm (Ω)
Energy E Erg (erg) Joule (j)
Frequency f Cycle/sec Hertz (Hz)
Heat Q Erg( erg) Joul( J)
magnetic field B gauss (gauss) tesla (tesla)
refractive index μ unit less unit less
Inductance L abhenry (abH) henry (H)
Momentum p g-m/s (g-m/s) kg-m/s kg-m/s
plane angle Θ radian( rad)
solid angle Ω steradian (sr)
pressure P bar (Dyne/cm²) pascal (N/m²) (Pa)
power P Erg / second watt (Joule / second (W)
Specific heat capacity c J kg−1 K−1
Wavelength λ Centimeter (cm) Meter( m)
Entropy S Joule/Kelvin (J K−1)
dynamic viscosity poise [g/(cm·s)] P Pascal-second Pa·s
Surface tension Y Dyne cm−1 / erg m−2 N m−1 or J m−2