A phenomenon is a unique or recurrent occurrence; a concept is something general. A phenomenon may be described; a concept may be defined. A phenomenon may be subsumed under a concept. The operationalization of a concept enables one to decide whether a given phenomenon is to be subsumed under it.

In particular, a historical change is a phenomenon individuated by spatio-temporal coordinates. The meaning of the expression emancipation of woman may serve as an example:

• It may refer to a historical phenomenon. This is then the emancipation of woman which occurred in some country X at a particular time Y in its history. In this case, the expression should read, more exactly, the emancipation of woman in country X at time Y. This phenomenon may be described, for instance by historians. It cannot be defined because it is a particular, not a general entity.
• Alternatively, the expression may designate the concept ‘emancipation of woman’. This can be defined; and then one may check whether the phenomenon that has been called the emancipation of woman in country X at time Y does or does not fall under the concept ‘emancipation of woman’.

The same goes for linguistic changes. The expression lenition designates the concept ‘lenition’. It may be defined as a phonological process and be viewed as a diachronic process. In the history of Spanish, Latin voiceless stops (/p t k/) changed into Spanish voiced stops (/b d g/), as in Latin vita ‘life’ and Spanish vida id. This is a phenomenon that can be described. It may then be seen whether it instantiates the concept ‘lenition’.

This distinction between concepts and phenomena excludes definitions like ‘lenition is a historical process’. What is meant by such definitions is commonly “lenition is a process which may manifest itself in diachrony”, so that certain historical processes may be instances of it.