(Science: zoology) One of the monotremata.
there are three different groups of mammals: placentals, marsupials and monotremes. Monotremes are generally considered to be a more primitive kind of mammal. They are warm-blooded, have fur, and produce milk to feed their young just like all mammals. The thing that is different about them is that they lay eggs instead of bearing live young. Some parts of their skeletons are more similar to a reptile than to a mammal.
More primitive kinds of mammals such as monotremes would generally have been expected to be outcompeted when more advanced mammals like the placentals evolved; but australia became an island when it split away from what is now Antarctica millions of years ago. It contained no advanced mammals at that time, and its island isolation prevented advanced placental mammals from finding their way there. With no competition from other mammals, the monotremes survived. It is for this same reason that australia contains so many marsupials, which are also considered to be a more primitive kind of mammal.