chalk

(uncountable) A soft, white, powdery limestone (calcium carbonate, CaCO3).

(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).

Tailor's chalk.

(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).

(US, military, countable) A platoon-sized group of airborne soldiers.

(US, sports, chiefly basketball, horseracing) The favorite in a sporting event.

(US, sports, chiefly basketball) The prediction that there will be no upsets, and the favored competitor will win.