Geometry can't be defined easily in one sentence.
Geometry evolved from the axioms and connections of space. Geometry is a Greek noun meaning survey. Its evolved branches are the economy system (aera- and volume counting), and astronomy. Later it wasn't built on experience, but axioms.
Like many sciences, it was also invented in Ancient Greece. For about two thousand years, the axioms of Euclidean geometry were the roots of all sciences. As a result of the work of Carl Friedrich Gauss, Nyikolaj Ivanovics Lobacsevszkij, Bolyai János, Henri Poincaré and Bernhard Riemann in the 1800s, the non-Euclidean geometry was invented.