Fossilization refers to errors that persist in adult language learners' speech even after years of exposure and practice. Examples include saying "I no understand" instead of using proper grammar. Whether fossilization has occurred depends on social identity factors and the need to communicate. Some debate exists around considering learners fossilized if they retain a foreign accent but are otherwise fluent, or if learners are unconcerned with sounding native. Progress should not be solely measured against native speaker norms.