A system which is homogeneous in composition, homogeneous in chemical aggregation and invariable in chemical aggregation is said to be pure substance. Homogeneous composition refers the composition of each part of the system is same as the composition of every other part. Homogeneous  chemical composition refers chemical element must be combined chemically in the same way in all parts of the system. Invariable chemical aggregation refers the state of chemical combination of system does not change with time.

A pure substance can have multiple phases. An  ice-water mixture is a pure substance. Air is being composed of a mixture of nitrogen, oxygen and other gases, formally not a pure substance. However, experience show that we can often be treated air as a pure substance with little error. An air-steam mixture is not a pure substance.