Speed of light symbol

An italic lower-case c, denoting the speed of light in vacuum.

Also known as: Light speed constant, Italic c

ScientificMathematicalStandard profile

Meanings

c is the speed of light in vacuum, and since 1983 its value is exact by definition — the metre is defined from it. Measuring the speed of light more precisely is now impossible in principle, because it would just be measuring the metre.

Common modern usageVerified fact
Denotes the speed of light in vacuum, the invariant speed at which massless particles travel and the upper limit on the propagation of any signal.
Contexts: Mathematics, Laboratory
Officially standardisedVerified fact
Has had an exact value, 299 792 458 metres per second, since 1983 — fixed by definition rather than measured, because the metre is now defined in terms of it.
Contexts: Mathematics, Laboratory
Common modern usageVerified fact
Is thought to derive from the Latin celeritas, meaning swiftness, though the attribution is not firmly established.
Contexts: Mathematics, History, Laboratory

Visual description

A lower-case Latin letter c set in italic type, an open curve facing right, distinguished from an upright c by its slant.

Symmetry: none
Orientation: upright

Commonly confused with: Degree Celsius symbol
Both reduce to a bare letter c and appear in the same scientific writing.
Part of the same system as: Planck constant symbol
Constants whose exact values define the SI since 2019.

Compare: Degree Celsius (°C) vs the speed of light (c) →

Confidence

IdentityHigh confidence: High confidence because the identity is defined by a current official standard, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is defined by a current official standard, but multiple independent sources agree.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references, but multiple independent sources agree.

Sources

The SI defining constants
Bureau International des Poids et Mesures · retrieved 2026-08-01
The seven constants whose exact values define the SI since 2019. Cited for the fact that a constant's value is fixed by definition rather than measured — a claim about the unit system, not about the notation.
CODATA Internationally Recommended Values of the Fundamental Physical Constants
NIST / CODATA Task Group on Fundamental Constants · retrieved 2026-08-01
Authoritative for the recommended value and uncertainty of a physical constant, and for whether that value is exact by definition or measured. It fixes values; it does not decide which letter denotes which quantity, and no record cites it for a symbol's meaning.
Speed of light
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
Physical constant
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.

Last reviewed 2026-07-31

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Physical constants and quantity symbols

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Meanings

Common modern usageVerified fact

Denotes the speed of light in vacuum, the invariant speed at which massless particles travel and the upper limit on the propagation of any signal.

Contexts: Mathematics, Laboratory

Officially standardisedVerified fact

Has had an exact value, 299 792 458 metres per second, since 1983 — fixed by definition rather than measured, because the metre is now defined in terms of it.

Contexts: Mathematics, Laboratory

Common modern usageVerified fact

Is thought to derive from the Latin celeritas, meaning swiftness, though the attribution is not firmly established.

Contexts: Mathematics, History, Laboratory

Confidence

IdentityHigh confidence: High confidence because the identity is defined by a current official standard, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is defined by a current official standard, but multiple independent sources agree.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references, but multiple independent sources agree.

Sources

The SI defining constants
Bureau International des Poids et Mesures · retrieved 2026-08-01
The seven constants whose exact values define the SI since 2019. Cited for the fact that a constant's value is fixed by definition rather than measured — a claim about the unit system, not about the notation.
CODATA Internationally Recommended Values of the Fundamental Physical Constants
NIST / CODATA Task Group on Fundamental Constants · retrieved 2026-08-01
Authoritative for the recommended value and uncertainty of a physical constant, and for whether that value is exact by definition or measured. It fixes values; it does not decide which letter denotes which quantity, and no record cites it for a symbol's meaning.
Speed of light
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
Physical constant
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.

Last reviewed 2026-07-31

Meanings

Visual description

Related & commonly confused symbols

Meanings

Visual description

Related & commonly confused symbols