say it: DOWN-sterz
downstairs is an adjective and adverb meaning "on or of lower floors of a building". It carries 2 recorded senses.
on or of lower floors of a building
“the downstairs (or downstair) phone”
on a floor below
“the tenants live downstairs”
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for downstairs come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.