What is the term differential used to indicate

It would help if we had a context. Are you doing differential analysis in cost/managerial accounting?

In that context, it's just meaning the difference between two options. "Differential" is just a fancy word.

The term differential is used to indicate the difference between the cost of the investment as recorded on the parent's books and the book value of the shares acquired based on the stockholders' equity accounts of the subsidiary.