belated Monday word: sandhi

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sandhi  (snd, sän-), noun

Modification of the form or sound of a word or morpheme under the influence of an adjacent word.

(A morpheme is the smallest grammatical unit in a language:  it has meaning, but no smaller part of it has meaning.  For example, 'unforgiveable' has three morphemes:  'un' and 'able' are bound morphemes, and 'forgive' is a free morpheme.)

One example of sandhi is the use of 'a' before words starting with a consonant (a hawk) and 'an' before words starting with a vowel (an owl).
Another is the phrase "cats and dogs", where 'cats' ends with an 's' sound but 'dogs' ends with a 'z' sound.
Those are both examples of external sandhi:  changes at the boundaries of words.
Internal sandhi is a change within a word; the Wikipedia example is 'sympathy' (syn- + pathy).

Etymology:  1800s, from Sanskrit samdhi, 'placing together'.  This sort of word modification is more common in Indian languages, and especially Sanskrit, hence the name.