Plain English Meaning
The aircraft is built the way it was approved to be built, and everything on it is currently working safely and up to date on inspections. If either of those is not true, it is not airworthy.
Full Definition
The state of an aircraft in which it conforms to its FAA type certificate (and any approved modifications) and is in a condition for safe operation. An aircraft is in airworthy condition only when both criteria are met: it matches its approved design, and all required inspections, maintenance, and component life limits are current.
Context Anchor
Pilots encounter this term during preflight checks, aircraft logbook review, maintenance discussions, and decisions about whether an aircraft may legally and safely be flown.