The present-value-weighted average time until all of a bond’s payments are received.
Frederick Macaulay defined it in 1938. For a zero-coupon bond it equals the maturity; for coupon bonds it is lower. Dividing it by (1 + yield) gives modified duration, which estimates the percentage price reaction to yield changes. If the investment horizon equals the Macaulay duration, a bond is largely immunised against rate changes.