a fast: deliberately going without food for a time to humble oneself before God in grief, repentance, or urgent need
A fast in the Bible is never a diet or a health practice. It is an act of the whole person turning toward God, usually in a hard moment: mourning a death, pleading for a sick child, confessing sin, or begging for rescue. Going without food is the outward sign of an inward posture the Old Testament calls afflicting or humbling oneself. It commonly travels with weeping, wearing rough sackcloth, and sitting in ashes.
The word covers a range of settings that shade into one another. A single person may fast in private grief or earnest prayer. A leader may summon a whole community to fast together when danger looms or the nation needs to repent. And after the exile, certain fasts became fixed days on the calendar, kept year after year to remember past disasters. All three are the same basic act, differing mainly in who fasts and whether the occasion is spontaneous or scheduled.
The prophets warned that the act can go hollow. Isaiah 58 pictures people fasting while they exploit their workers and quarrel, and God rejects it: the fast he wants is to feed the hungry and free the oppressed. So the word carries a built-in tension. It names a genuine expression of devotion, and also the very ritual most easily faked or emptied of meaning.