In historical linguistics and language contact, unpacking is the separation of the features of a segment into distinct segments.
Perhaps the most common example of unpacking is the separation of nasal vowels into vowel plus nasal consonant when borrowed into languages which don't have nasal vowels. This can be seen in English borrowings of French and Portuguese words, such as monsoon [mɒnˈsuːn] from Portuguese monção [mõsɐ̃ũ], but occurs widely, as in Lingala [balansi] from French [balɑ̃s] "balance". Here the nasality of the vowel is separated out as a nasal consonant. If this didn't happen, the nasality would be lost.
Unpacking occurs not just in borrowings, but within a language over time. Sanskrit syllabic ऋ [r̩] has become [ɾɪ] in Hindi, which has no syllabic consonants; the rhoticity is maintained by the [ɾ], while the syllabic feature is separated out as a vowel.
The opposite of unpacking is fusion.