See the specification for <i>, a taxonomic designation is the first example of its use.

https://www.w3.org/TR/html5/text-level-semantics.html#the-i-...

Well, I should specify I meant that in most cases using <em> is preferred over <i> in terms of the traditional sense of what <i> was used for in the past. These days both have their specific uses, but in most cases one would more likely to use <em>.

For example, further down the page:

"Authors are encouraged to consider whether other elements might be more applicable than the i element, for instance the em element for marking up stress emphasis, or the dfn element to mark up the defining instance of a term."

As an element it used to be the way to italicize text, to emphasize that text, before CSS. These days it has had a semantic reason for its existence applied which is a slight variation of its original purpose under the HTML4 spec.