A morphosyntactic property is a grammatical property of a grammatical word. These properties are manifested as exponents in phonological words. Examples of MSPs are PLURAL, FIRST PERSON, PAST PERFECT, and COMPARATIVE.
The more flexible MSP/exponent model is now often used in place of the older morpheme/morph model. The previous idea had problems in that morphs don't form a natural class in a language (that is, there are never any rules like "Remove the first morph from a word") and that abstract meanings don't always translate into morphs in any direct, one-to-one way.