Asubstance that will change shape to fit its container but has a definite volume is in phase of matter.

A substance that will change its shape to fit its container, but has a definite volume, is in a liquid state of matter.

There are four (confirmed) states of matter, Solids, Liquids, Gasses, and Plasma. Solids, have definite shape and volume. Metalloids is a solid that does not have a definite shape, but does have an 'original' shape, the shape the metalloid was originally in. Liquids although have no original shape, although is does have a definite volume, which is how much space it takes up. Liquids will change to fit the shape of its container, so no matter what you put a liquid in, it will fit its container in some way or form. Water for example, being a liquid at room temperature, when pouring water into a bottle, you see it take the shape of the container, as supposed to. Gasses and Plasmas are a bit more complicated, so you could look those up if you'd like. I hope this helps you.