What state of matter has neither a definite size nor a definite shape?

The state of matter that has neither a definite size nor a definite shape is gas. Gases are characterized by their ability to expand and fill the volume of any container they occupy, which means they take on the shape of the container. In this state, the particles are much farther apart compared to solids and liquids, moving freely and rapidly, which allows them to spread out over a larger area. This lack of fixed size and shape is a fundamental property of gases and distinguishes them from solids, which maintain a definite shape and volume, and liquids, which have a definite volume but take the shape of their container without a fixed shape. Plasma, while also lacking a definite shape and size, is a state of matter that consists of ionized gas with free electrons, found in situations like stars and lightning. However, the traditional understanding of gases as having no fixed size or shape is most directly aligned with the answer provided.