The Spanish word 'leche' meaning 'milk' comes from the Vulgar Latin word 'lacte', which was the accusative form of Classical Latin 'lac', also meaning 'milk'. The evolution from Latin 'lac/lacte' to Spanish 'leche' shows typical sound changes that occurred as Latin evolved into Spanish, including the softening of 'ct' to 'ch' and the modification of the vowel 'a' to 'e'.

Several common Spanish words are derived from the same Latin root 'lac/lacte', including 'lácteo' (dairy), 'lactancia' (lactation), and 'lechería' (dairy shop). These words maintain their connection to the basic meaning of milk and dairy products.

English speakers can recognize this etymology in words like 'lactose', 'lactate', and 'lactic acid', all of which come from the same Latin root 'lac'. The scientific and medical terms in English have preserved the original Latin spelling with 'lact-', while Spanish evolved the everyday word to 'leche'.