Boundary

A boundary is a formal device used in Chomsky & Halle (1968) to express a distinction between two types of affixes.

Examples
''(i) prod&uacutective   product&iacutevity  prod&uacutectiveness op[ei]que   op[æ]city	   op[ei]queness''

The claim is that -ity is a morpheme-boundary or formative-boundary affix (i.e. +ity), and -ness a word-boundary affix (i.e. #ness).

Comment
The assumption that affixes are associated with different boundaries, viz. + (morpheme boundary) and # (word boundary) accounts for the fact that the English suffixes -ity and -ness behave differently with respect to a number of phonological rules, as shown in (i).

Link
Utrecht Lexicon of Linguistics