GPS
Definition
The Global Positioning System — the satellite network your phone uses to figure out exactly where on Earth you are, by timing radio signals from space.
Technical definition
A satellite navigation system in which receivers determine position by trilateration: measuring signal travel times from at least four satellites and multiplying by the speed of light. Nanosecond timing demands atomic clocks aboard satellites plus relativistic corrections.
Example
Light travels about 30 cm in one nanosecond, so a mere 10-nanosecond clock error would misplace you by 3 meters. GPS satellite clocks are corrected for relativity — without it, positions would drift by roughly 10 km per day.
Your phone does not transmit to satellites to get located — GPS is receive-only; the phone silently listens to satellite broadcasts and computes its own position. And GPS needs a clear sky view because the signals are weak and easily blocked, which is why indoor positioning is poor.