sólo

Solo functions as both an adverb ("only, just") and an adjective ("alone"). As an adjective it agrees with the noun in gender and number: solo (masc sg), sola (fem sg), solos (masc pl), solas (fem pl). "Una mujer sola" means a woman alone; "los ninos estan solos" means the children are alone. As an adverb it is invariant: "solo quiero agua" means "I only want water".

The Real Academia Espanola dropped the written accent on solo when it means "only" (formerly solo vs sólo), so context disambiguates the two senses. Modern style guides use solo across the board.