A fundamental (base) unit is an independently defined unit of a fundamental quantity. The SI system defines 7 base units: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), candela (cd), plus 2 supplementary units: radian (rad) for plane angle and steradian (sr) for solid angle.

• In mechanics, only 3 fundamental quantities are needed (mass, length, time), but this choice is not unique — any 3 independent mechanical quantities could serve as fundamental. If speed and time are chosen, length becomes derived.
• The SI system extends MKS by adding ampere, kelvin, mole, and candela to cover electromagnetism, thermodynamics, chemistry, and photometry respectively.
• Trap: the unit symbol for kelvin is uppercase K, not lowercase k. Lowercase k is the SI prefix for kilo (10³). Confusing these on a NEET answer sheet changes the physical quantity entirely.

A derived unit is expressed as a product of fundamental units raised to appropriate powers. Key decompositions: velocity = m·s⁻¹, acceleration = m·s⁻², force (N) = kg·m·s⁻², energy (J) = kg·m²·s⁻², power (W) = kg·m²·s⁻³, pressure (Pa) = kg·m⁻¹·s⁻².

• To convert a derived unit between systems, decompose into base units and convert each factor independently: 1 J = 1 kg·m²·s⁻² = (10³ g)·(10² cm)²·s⁻² = 10⁷ erg.
• Every derived unit can be expressed using exactly 7 SI base units with integer or fractional exponents — this expression is the dimensional formula of the unit.
• Trap: a light year contains the word year but measures length (distance light travels in one year ≈ 9.46 × 10¹⁵ m). NEET uses this naming confusion as a distractor — always check what physical quantity a unit measures.