Fossilizable linguistic phenomena are linguistic items, rules and subsystems which speakers of a particular NL will tend to keep in their IL relative to a particular TL, no matter what the age of the learner or the amount of explanation and instruction he receives in the TL.
Doughty highlights this essential difference between L1 and L2 acquisition:
Given adequate exposure, normal intelligence and normal social conditions, children can be expected to learn the language(s) of their caregivers incidentally and fully, such that they are eventually indistinguishable from other native speakers of their speech community. In stark contrast, language acquisition by adults is guaranteed only to be variable both within and across individuals, most typically relatively unsuccessful, and always incomplete, such that non-native speakers can be invariably identified as such, provided judgements are made on adequate samples of performance.
Regarding the term fossilisation, Long (2003) prefers the term stabilisation which he considers synonymous apart from the characteristic of permanence, which he sees as difficult to operationalise.
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