A split infinitive occurs when a word or group of words is inserted
the two parts of an infinitive.
Abe

Split infinitives occur when an adverb is inserted between two parts of an infinitive.

Explanation:

By grammatical rules, inserting an adverb between the two parts of infinitive is non permissible. But in normal usage it has been around liberally, and often in ornate, literary language.

For example, an infinitive like 'to go' has been used often by

'to boldly go'

'to gracefully go'

These are proper usage phrases that occur often. a word added in between added to the general meaning of the phrase given here.