Can there be a comma under the definition?

Of course. Whether there can be commas in the definition depends on the length of the definition. If it is long, there can be commas. A formula should be remembered when defining:

The so-called definition is to prompt the connotation of the concept with short and clear sentences, that is, to reveal the characteristics or essence of the object reflected by the concept. Expressed as:

Defined concept = species difference+adjacent generic concept ("species difference" refers to the unique attribute of a species concept under the same generic concept (essential difference from other generic concepts), and "adjacent generic concept" refers to the smallest generic concept that contains defined people.

For example, folk songs are poems written by working people, which directly express their thoughts, feelings and demands.

In this definition, "poetry" is an adjacent concept. The essential difference between folk songs and other poems is that they directly express the thoughts, feelings and demands of working people and are created by working people. That is, planting differences.