is a highly reactive chemical species with one or more unpaired electrons

Explanation:

A specie with no charge and containing one or more number of unpaired electrons is known as a free radical.

These free radicals tend to attain stable configuration by gaining an electron from another atom. Hence, they fill their outer energy levels.

For example, a free radical of chlorine will readily combine with a hydrogen atom in order to attain stability and it leads to the formation of a new chemical bond between the hydrogen and chlorine atom.

As a result, it will form a hydrogen free radical.

Thus, we can conclude that free radical is a highly reactive chemical species with one or more unpaired electrons.

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