Definition

Resistance

R \equiv \frac{Δ V}{I}

Statement

Volts required per ampere delivered.

Symbols and units

When it applies

Any two-terminal element, ohmic or not.

Derivation

A definition, applied to any two-terminal element:

R \equiv \frac{Δ V}{I}

For some materials R happens to stay constant as ΔV changes. That extra claim is Ohm's law — the definition itself is always valid:

Δ V = I R (ohmic only)

Watch out for

Keep the two apart. A diode has a well-defined R at each operating point but does not obey Ohm's law, since R changes with voltage.

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