i+1

Etymology
Ostensibly in his Explorations in Language Acquisition and Use in 2003; but possibly earlier. Originally, referred to a subsequent word (the 1) added to an existing vocabulary of i words, i.e. the i+1th word learned by the learner.

Pronunciation
The same as I plus one.

Adjective

 * 1)  Containing only a single word unknown to the listener or reader.

Noun

 * 1)  The existence of only one word unknown to the reader or listener of a sentence.