Requirement for quality: Expression of the needs or their translation into a set of quantitatively or qualitatively stated requirements for the characteristics of an entity to enable its realization and examination.

Radiation: A method of transmission of energy.

Range: (1) Extent of coverage of effectiveness. (2) Measure of distance.

Ratio Bridge: A bridge circuit that uses a calibrated resistive or calibrated inductive voltage divider for one side of the bride.

Reference Line: (A line from which all other measurements are taken.

Reference Plane: A reference lie that has been rotated through 360 degrees.

Repeatability: Same reading each time for the same solution.

Resonance: An excited state of a stable particle causing a sharp maximum in the probability of absorption of electromagnetic radiation.

Restoring Force: The constant mechanical force provided.

RHO: The magnitude of the reflection coefficient.