The answer would be: body cells that have been infected
Cytotoxic comes from word cyto- which mean cells and toxic which mean poison. That means cytotoxic cells will be poisonous to the cells. Cytotoxic t cells roles are to attack cells that infected by pathogens. Some pathogens hide inside the cells so that they won't captured by the leucocytes.
Cytotoxic t cells destroy such cells so the pathogens inside them could be killed.