Plural of atrium; either the upper chambers of the heart that receive blood, or large open-air courtyards in buildings.
From Latin 'atrium' (main room of a Roman house), which originally meant the central court. The anatomical usage transferred metaphorically because of the chamber's role in receiving blood, like the room receives visitors.
Roman houses had atriums as their heart—a central open courtyard where families gathered and light entered—and interestingly, your heart's atriums do almost the same thing: they're the 'gathering chambers' where blood collects before being pumped out.
Complete word intelligence in one call. Free tier — 50 lookups/day.