Definite volume, no definite shape. Milk is a liquid. Soda is a liquid. Matter that has no definite shape and no definite volume is a gas . For example, air has no definite volume and will change shape according to the things around it.
Air is a gas.
Oxygen is a gas. It is one of the gases in the air. A gas will spread out to fill up the spaces around it. So it has no definite shape of its own. A gas will enlarge by spreading out if there is lots of room.