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(uncountable) A soft, white, powdery limestone (calcium carbonate, CaCO₃).
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(countable) A piece of chalk, or nowadays processed compressed gypsum (calcium sulfate, CaSO₄), that is used for drawing and for writing on a blackboard (chalkboard).
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Tailor's chalk.
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(uncountable, climbing, gymnastics) A white powdery substance used to prevent hands slipping from holds when climbing, or losing grip in weight-lifting or gymnastics, sometimes but not always limestone-chalk, often magnesium carbonate (MgCO₃).
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(US, military, countable) A platoon-sized group of airborne soldiers.
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(US, sports, chiefly basketball, horse racing) The favorite in a sporting event.
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(US, sports, chiefly basketball) The prediction that there will be no upsets, and the favored competitor will win.