The Spanish word 'penumbra' comes from combining two Latin words: 'paene' meaning 'almost' and 'umbra' meaning 'shadow'. This compound literally means 'almost shadow', which perfectly describes its modern meaning - the partial or lighter shadow that occurs in an eclipse or around the edges of a complete shadow. The term was actually coined in the 17th century to describe astronomical observations, but has since been adopted into both Spanish and English with the same meaning.

A simpler related Spanish word is 'sombra' (shadow), which also derives from Latin 'umbra'. While 'penumbra' refers specifically to a partial shadow, 'sombra' is the general word for any shadow or shade. You can see how both words preserve the 'umbra/ombra' root relating to shadows.

English speakers might recognize this word easily since 'penumbra' exists in English with exactly the same spelling and meaning. Additionally, we have several other words from Latin 'umbra', such as 'umbrella' (originally meant for shade from the sun), 'somber' (dark and gloomy), and 'umbrage' (shadow or shade, though now more commonly used in the phrase 'take umbrage' meaning to take offense). The prefix 'pen-' (from Latin 'paene' meaning 'almost') appears in English words like 'peninsula' (almost an island) and 'penultimate' (almost the ultimate or last).