Two or more segments are tautosyllabic (with each other) if they occur in the same syllable. For instance, the English word "cat", /kæt/, is monosyllabic and so its three phonemes /k/, /æ/ and /t/ are tautosyllabic. They can also be described as sharing a 'tautosyllabic distribution'.
Phonemes that are not tautosyllabic are heterosyllabic. For example, in the English word "mustard" /ˈmʌstərd/, /m/ and /t/ are heterosyllabic since they are members of different syllables.