of matter, states of the same substance (for example, water, iron, sulfur) the transitions between which are accompanied by discontinuous changes in free energy, entropy, density, and other fundamental physical properties.

What is meant by state of aggregation?

: one of the three or more fundamental forms, conditions, or states of matter that are commonly considered to include the solid, liquid, and gaseous forms and often others (such as the colloidal)

What are the usual states of aggregation of matter?

The matter is present in the nature in three different States of matter aggregation: solid, liquid and gas. Solids have fixed shape and volume. Liquids have fixed volume, but its shape conforms to that of their container.