An injury or mark where the skin has not been broken but is darker in colour, often as a result of being hit by something. (Cambridge Dictionary)
A bruise is an injury which appears as a purple mark on your body, although the skin is not broken. (Collins Dictionary)
An injury appearing as an area of discoloured skin on the body, caused by a blow or impact rupturing underlying blood vessels. (Oxford Dictionaries)
To inflict an injury involving rupture of small blood vessels and discoloration without a break in the overlying skin or a similar injury to plant tissue : to inflict a bruise. (Merriam – Webster)
How to Treat a Bruise.