litre UK /ˈliːtə/US /ˈli.tɚ/ noun3 •The metric unit of fluid measure, equal to one cubic decimetre. Symbol: L, l, or ℓ.You should be able to fill four cups with one litre of water. •(informal) A measure of volume equivalent to a litre. •Lithraea caustica, a Chilean tree or bush. formslitres