When there is not enough food to feed everybody. Usually used for humans, but other animals also suffer.
Famine can be caused by naturally (by
drought or excessive rain, by diseases or other factors) or can be man-made (by war or political matters) or even a mix of both (those in power take advantage of the food
shortage for their own gain).
Natural case point: Europe's Great Famine of 1315-1317, when a spell of cold weather and heavy rains ruined crops, driving people to infanticide, child abandonment, and cannibalism.
Man-made case point: the
Holodomor in 1932 Ukraine, when Stalin's policies of collectivism
starved up to ten million Ukrainians, driving some to cannibalism to survive. Most of the victims were peasants who had their crops, livestock, and food stolen from them pointblank by "activist" soldiers.
Mixture case-point:
Irish potato famine (An Gorta Mor) of 1845-1849, when a blight ruined potato crops and excess grain exports on the part of British authorities starved over a million Irish peasants and caused a similar number of people to flee from Ireland.
The Donner Party, whose poor choice of shortcuts cost them many resources and precious days before winter, suffered their own famine in the Sierra Nevada mountains.