Interlanguage is the term for an ideolect that has been developed by a learner of a second language (or L2) who has not yet reached proficiency. A learner's interlanguage preserves some features of their first language (or L1), and can also overgeneralize some L2 writing and speaking rules. These two characteristics of an interlanguage result in the system's unique linguistic organization.
An interlanguage is idiosyncratically based on the learners' experiences with the L2. It can "fossilize", or cease developing, in any of its developmental stages. The interlanguage rules are claimed to be shaped by several factors, including L1-transfer, previous learning strategies, strategies of L2 acquisition (i.e., simplification), L2 communication strategies (i.e., circumlocution), and overgeneralization of L2 language patterns.