Quotations in English papers are used to directly quote words and expressions from other resources.

You can punctuate this sentence like this: in the era of $ TERM axis, the word "axis" means pivot. Although shaft age is also quoted, it is a term without quotation marks. As for the last word pivotal, if this explanation is given by others, it needs to be quoted. If it is your own understanding, you don't need to put quotation marks.