PIE base ‘to measure’. See also *med- ‘to take appropriate measures’, PIE root of medicine.
Derivatives include meter, metre, metric, symmetry, geometry, piecemeal, meal, semester, moon, month, menstruate, and menopause. Also root of Greek metron, from which metronome is derived. In contrast, metropolis means ‘mother city’, from Greek mētēr ‘mother’, from PIE base *māter- ‘mother’.
[Pokorny *mē-3, m-e-t-, pp. 703–704.]