Plain English Meaning
Information sent down by each GPS satellite saying 'here is exactly where I am right now.' Your GPS receiver uses that, along with similar messages from other satellites, to work out where you are.
Full Definition
In GPS, ephemeris data is the precise orbital information broadcast by each GPS satellite that tells a receiver exactly where that satellite is at a given moment. The receiver needs this position data, combined with timing signals, to calculate its own location.
Context Anchor
Seen in GPS operation discussions, especially when explaining how a GPS receiver finds satellites and calculates position.
Derivation
From the Greek 'ephemeros,' meaning 'lasting only a day' or 'daily.' Astronomers used 'ephemeris' for tables giving the daily positions of celestial bodies. GPS borrows the term because each satellite is, in effect, a moving celestial object whose position must be known precisely at every moment.