وهو: ضم كلمة أو ما يجري مجراها إلى أخرى بحيث يفيد المخاطب أن مفهوم إحداهما ثابت لمفهوم الأخرى، أو منفي عنه
Predication joins a word, or what functions as one, to another so that the addressee understands one meaning as affirmed of or denied of the other.
Joining is not mere juxtaposition: it establishes a reportable relation. The musnad ilayh is that about which something is predicated; the musnad is what is predicated.
In «زَيْدٌ قَائِمٌ», standing is affirmed of Zayd. In «زَيْدٌ لَيْسَ بِقَائِمٍ», it is denied. The meaningful relation is what makes each an instance of isnād.