经度是一种用于确定地球表面上不同点东西位置的地理坐标。经度是一种角度量，通常用度来表示，并被记作希腊字母λ。子午线穿过南极和北极并把相同经度的点连起来。按照惯例，本初子午线是经过伦敦格林尼治皇家天文台的子午线，是0度经线所在地。其他位置的经度是通过测量其从本初子午线向东或向西经过的角度得到的，经度的范围为从本初子午线0° 向东至180°E 和向西至180° W。具体来说，某位置的经度是一个通过本初子午线的平面和一个通过南极、北极和该位置的平面所组成的二面角。(这就组成了一个右手坐标系，其z轴(右手拇指)从地球中心指向北极方向，其x轴(右手食指)从地球中心指向本初子午线与赤道的交点。)

Longitude (/ˈlɒndʒᵻtjuːd/ or /ˈlɒndʒᵻtuːd/, British also /ˈlɒŋɡᵻtjuːd/), is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians (lines running from the North Pole to the South Pole) connect points with the same longitude. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of zero degrees longitude. The longitude of other places is measured as the angle east or west from the Prime Meridian, ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Specifically, it is the angle between a plane containing the Prime Meridian and a plane containing the North Pole, South Pole and the location in question. (This forms a right-handed coordinate system with the z axis (right hand thumb) pointing from the Earth's center toward the North Pole and the x axis (right hand index finger) extending from Earth's center through the equator at the Prime Meridian.)