What does 'interlanguage' mean in second language learning?

According to Stephen Krashen's Input Hypothesis, providing 'comprehensible input' in second language learning implies that instruction should transition from:

Positive transfer (or positive interference) of learning helps in the acquisition of the ____

What is the term coined by Stephen Krashen to describe a stage where a learner is at stage 'i' and language acquisition takes place when they are exposed to a slightly higher level at stage 'i + 1'?