A model of phonological change in which a sound innovation spreads gradually through individual lexical items across the vocabulary rather than applying synchronously to all tokens of an affected phoneme; diffusion typically produces a mosaic of innovated and conservative forms conditioned by lexical, phonological and social factors.

Principle

Phonological change can proceed item-by-item: adoption rates vary by word frequency, morphological status, phonological neighborhood and speaker/community factors, so an innovation may appear in some words earlier than in others and may stabilize unevenly across the lexicon.

Demonstration

Illustrative scenario → Situation: a community develops a new pronunciation variant for a vowel. Recognition: corpus or field sampling shows the variant in a subset of words (often higher-frequency or certain morphological classes) while other words retain the older pronunciation. Action: researchers map diffusion across words and correlate spread with lexical properties. Consequence: the language exhibits mixed variant distributions and lexical exceptions rather than an immediate, exceptionless phonological rule.

Misapplication

Mistaken interpretation: treating any non-uniform distribution of variants as evidence of diffusion. Semantic error: ignoring alternative explanations such as phonological conditioning, morphophonological reanalysis, analogical leveling, or recent borrowing that can produce non-uniform patterns.

Consequence

Predicts and explains staggered adoption of variants, persistent lexical exceptions, and complex distributions relevant to historical reconstruction, dialectology and models of speech processing; misapplying it can mislead reconstructions or sociophonetic interpretation.

Reversal

Qualification: Some sound changes operate as synchronically active phonological rules that apply across the system (near‑exceptionless), or are rapidly regularized by analogy or morphological restructuring, in which case the lexical-diffusion model is not the best description.

Boundary

Clearly within: a sound change attested to affect some lexical items but not others with no single phonological environmental conditioning. Boundary case: a change that is partly phonologically conditioned and partly lexicalized. Clearly outside: an automatic phonological rule that applies uniformly given the same phonetic environment.

Semantic Tension

Lexicon‑level Spread ↔ Systematic Rule — the tension between item-by-item lexical spread and systemwide rule application constrains how change is modeled and explained.

Synthesis

Lexical diffusion emphasizes that phonological change often begins at the level of individual words and that the interface of lexical properties, phonetic motivation and social dynamics determines whether an innovation remains lexicalized or generalizes into a system-level rule.