WGS84 (World Geodetic System 1984) is the global datum and reference ellipsoid that GPS and most web mapping rely on. It defines an Earth-centered, Earth-fixed coordinate frame and an ellipsoid (semi-major axis 6,378,137 m, flattening 1/298.257223563) against which latitude, longitude, and ellipsoidal height are expressed.
Why it matters
Almost all field GPS receivers, satellite imagery products, and online basemaps deliver coordinates in WGS84. Because it is a single global datum, it lets data from different sources line up without a datum transformation — as long as everyone is genuinely on WGS84 and not a regional datum that looks similar. Its most common geographic form is identified by EPSG:4326, which stores positions as decimal-degree latitude/longitude.
A concrete example
A handheld GPS waypoint logged as 48.8584° N, 2.2945° E is in WGS84 (EPSG:4326). To measure distances or areas on it correctly you would project it into a metric system such as a UTM zone (for that location, EPSG:32631, WGS84 / UTM zone 31N). The underlying datum stays WGS84; only the projection changes.
Common pitfall
WGS84 is a datum, not a projection. Calculating area or buffer distances directly in EPSG:4326 produces wrong results because the units are degrees, not meters, and a degree of longitude shrinks toward the poles. Another frequent confusion: assuming WGS84 and NAD83 are interchangeable. They agreed at the centimeter level in the 1980s but have since diverged by more than a meter in North America because of plate motion and datum realizations.