Lambda (written λ, in lowercase) is a non-SI unit of volume equal to 10⁻⁹ m³, 1 cubic millimetre (mm³) or 1 microlitre (μL). Introduced by the BIPM in 1880,^[1] the lambda has been used in chemistry^[2] and in law for measuring volume, but its use is not recommended.^[3]

This use of λ parallels the pre-SI use of μ on its own for a micrometre and γ for a microgram.^[4][5] Although the use of λ is deprecated, some clinical laboratories continue to use it.^[6] The standard abbreviation μL for a microlitre has the disadvantage that it can be misread as mL (a unit 1000 times larger). In pharmaceutical use, no abbreviation for a microlitre is considered safe. The recommended practice is to write "microlitre" in full.^[7]