Plain English Meaning

A number with no units. It is just a ratio, like 'three times as much' or 'half as big.' It does not come with pounds, feet, knots, or anything else attached.

Full Definition

Describes a number or ratio that has no units of measurement attached to it, because the units cancel out when one quantity is divided by another of the same kind. A dimensionless value expresses a pure relationship between two things rather than a measured amount.

Context Anchor

Seen in aerodynamics and performance discussions, especially when comparing lift/drag ratio and other ratios in the Pilot’s Handbook of Aeronautical Knowledge.

Derivation

From 'dimension' (a measurable extent like length, weight, or time) plus the suffix '-less' (without). So dimensionless literally means 'without measurable units.' This helps because in physics the word 'dimension' refers not to space but to the type of unit a quantity carries.