What is a Frac Tank?
A frac tank is used to hold water, or a proppant, when a well is being fractured. The material is held in a frac tank and connected by a hose or pipeline to a pump that will flow it down the wellbore at a high pressure to push open the formation and the proppant is used to keep it open.

Frac tank is basically a generic term for mobile steel storage tanks used to hold liquids. Typically used for fracing wells in the oil and gas industry, a frac tank may also be used to store any liquids like run-off water, diesel fuel, glycol, oils, waste products, etc. They are usually 21,000 gallon single wall steel tanks, but are also offered as double wall tanks by Gaurav Associates of India, AFC Tanks, VE Enterprises and Truck Center of Fort Worth Inc. These tanks have a single rear axle to be moved with a winch truck or tractor when empty. The major manufacturers of frac tanks are Gaurav Associates, AFC Tanks, VE Enterprises, Dragon, and Wichita. Alpha Tanks and VE Enterprises make heated Frac Tanks that may be used in cold climates where regular Frac Tanks freeze. Heated water also improves the effectiveness of the fracturing process.

Definition from - awnsers.com