The rationale for this designation is obvious: when you suspend something from the ceiling while it is cut off below, it can never reach the ground. In the same way, a muʿallaq (suspended) report is one whose chain has lost its opening link.

Does the muʿallaq belong to the sound category or to the weak?
We say that, in principle, it is weak, for continuity of the isnād is a prerequisite for soundness. Nevertheless, any report that al-Bukhārī cites in muʿallaq form while stating it with certainty is deemed authentic in his judgment, even if it does not meet his usual conditions. We say “authentic in his judgment” because he only cites it in that manner as evidence for a legal ruling, and he would never derive a ruling for Allah’s religion from anything he did not consider authentic. Still, it remains outside his normal criteria, for had it satisfied them he would have transmitted it with its full isnād so that it might be known. Indeed—may Allah have mercy on him—he occasionally presents the same report as muʿallaq in one chapter and with a complete chain in another.

The second type—where the break occurs at the end of the chain—is called mursal.
The third type—where a single narrator is omitted from the middle of the chain—is, in technical usage, termed munqaṭiʿ; that is, a munqaṭiʿ report loses only one transmitter from its middle.
The fourth type—where two or more consecutive narrators are missing from the middle—is known as muʿḍal.