An abacus is an to manual calculation. The word is derived from Greek, and the Greeks were first to use 'counting tables' as calculating aids. The Chinese had an early abacus at about 500 BC, but the abacus we use today dates from about 1300 AD and can be used to add, divide, subtract, multiply and find square roots.
In spite of the presence of cheap calculators and computers, the abacus is still popular, especially in Asia. They are many arithmetic competitions using the abacus, and it is still widely used in schools as a teaching aid.