In second language development, a plateau where progress toward native-like mastery stalls is fossilization. This happens when a learner’s interlanguage—the evolving, personal grammar of the learner—settles into a stable pattern and certain errors become persistent across contexts, even when the learner continues to be exposed to the language and practice regularly. Think of interlanguage as a dynamic system that adjusts with new input and feedback. Fossilization means parts of that system stop changing. For example, a Spanish learner might consistently omit agreement markers or persist in using a non-target tense in everyday speech, despite years of study and exposure. Those features have become entrenched rather than corrected through continued practice alone. Fossilization can arise when feedback on those forms is infrequent or not sufficiently explicit to trigger reorganization of the learner’s rules, when the learner has limited opportunities to use the target forms in meaningful, corrective contexts, or when transfer from the learner’s first language reinforces non-target patterns. Affective factors like anxiety or low motivation can also play a role. To help a learner move beyond fossilization, instruction often needs targeted form-focused work, explicit explanations of the rules, corrective feedback that highlights inaccuracies, and ample authentic practice that pushes the learner to produce the target forms in varied, meaningful contexts.