The hectare ( /ˈhɛktɛər/ or /ˈhɛktɑr/; symbol ha) is a metric unit of area defined as 10,000 square metres (100 m by 100 m), and primarily used in the measurement of land. In 1795, when the metric system was introduced, the 'are' was defined as being 100 square metres and the hectare ('hecto-' + 'are') was thus 100 'ares' or 1/100 km². A hectare of land is about 2.47 acres.

When the metric system was further rationalised in 1960, resulting in the International System of Units (SI), the are was not included as a recognised unit. The hectare is one of the non-SI units accepted for use with the SI units.

The metric system of measure was first given a legal basis in 1795 by the French Revolutionary government. The law of 18 Germinal, Year III (7 April 1795) defined five units of measure:

Although the law defined the length of the metre, there was no practical way of accurately measuring the metre (and hence the are) until 1799 when the first standard metre was manufactured and adopted.

The standard metre remained in the custody of successive French governments until 1875 when, under the Convention of the Metre, its supervision passed into international control under the auspices of the General Conference on Weights and Measures (GCPM). At the first meeting of the GCPM in 1889 when a new standard metre, manufactured by Johnson Matthey & Co of London was adopted, the are and hectare were automatically redefined.