An object with one function, ideally, designed to be extremely good at what it does. A waffle iron, for example, can only be used to make waffles, but it tends to make waffles extremely effectively, justifying its existence as a uni-tasker. Objects as varied as saddles to soap dishes or rice cookers can be considered uni-taskers.
Mike loved unitasking despite never meeting his targets because of it. Each time someone talked to him he had to switch off his computer in case he would multitask by accident.