winter squash a name which is applied to those kinds of squash which are allowed to mature on the vine, then stored for use in winter. The skin is hard and inedible, while the inside becomes firm and is often superior in flavour to most sorts of summer squash.
The dividing line between summer squash and winter squash thus corresponds to differences in use. It does not correspond to divisions between the four principal botanical species of squash. Nor is it a firm dividing line, because there are some cultivars which can be harvested early as summer squash or kept to become winter squash.