litro
/ˈlitɾo/ · B1 · noun · m
liter
🧴🥛 litro = litre; both come from French litre, from Greek litra, a unit of weight
Swap the English -re (or US -er) for -o and you have the Spanish: litre becomes litro. It is masculine, so it takes el: el litro, un litro de leche.