Plural of jam; fruit preserves, or third person singular of jam meaning to become stuck or to pack tightly together.
Multiple origins: the preserve sense from French 'jame' meaning leg (from the crushing action); the blocking sense possibly imitative of the sound of impact. Both meanings converged in English by the 18th century.
The difference between jam, jelly, and preserves lies in fruit content - jam contains crushed fruit, jelly is made from fruit juice, and preserves have whole fruit pieces. Traffic jams got their name because vehicles become 'jammed' together like fruit in a jar.
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