Eggcorn
An idiosyncratic substitution of a word or phrase for a word, or words, that sound similar or identical in the speaker's dialect. The new phrase introduces a meaning that is different from the original but plausible in the same context.
An eggcorn can be described as an intra-lingual phono-semantic matching, a matching in which the intended word and substitute are from the same language. Together with other types of same-sounding phrases, eggcorns are sometimes also referred to "oronyms".
e.g. "old-timers' disease" for "Alzheimer's disease"