Definition of mu’allaq:[1] a drop at the beginning of the sanad.
For example, if Imām Bukhāri dropped his teacher it would be considered a muallaq narration. It does not matter if 1, 2, 3 or more narrators are dropped, such as the aḥādīth in Mishkāt al-Maṣābīḥ,[2] all of these narrations will be considered to be mu’allaq.
[1] Literally; hanging or tied.
[2] Compiled by Khatīb al-Tabrīzī (d. after 737)