contrast.solo-solamente
Solo, solamente, and solo the adjective
Solo (adverb) = only, interchangeable with solamente: solo quiero agua. Solo/a (adjective) = alone: vive sola. Since 2010, no accent needed on the adverb.
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Explanation
Solo plays two roles. As an adverb it means 'only' and is interchangeable with solamente: solo quiero agua = solamente quiero agua. As an adjective (solo/sola/solos/solas) it means 'alone' and agrees with the noun: vive sola, estaban solos.
Since the 2010 spelling reform, the adverb 'only' is written without an accent — solo, not sólo. In rare, genuinely ambiguous cases the RAE still permits the accent, but the modern default is accent-free throughout.
Ambiguity is rarer than it seems, because context decides: trabaja solo could be 'works alone' or 'only works', but the situation almost always makes it clear. Near-synonyms of 'only' include únicamente and, very colloquially in Latin America, nomás (pasá nomás, 'just come in'). A solas means 'in private': hablemos a solas.
Examples
Only your signature is missing.
Region: global
She prefers to work alone.
Region: global
I only wanted to let you know; I'd rather talk about it in private.