Al-munkar

A munkar report is one transmitted by a weak narrator that conflicts with another narrator who is even weaker than he is.

Al-maʿrūf

The opposite of the munkar is the maʿrūf.

Ruling on the maʿrūf, the munkar, the shādh and the maḥfūẓ

In both the maʿrūf and the munkar the transmitters are weak, and one of them is weaker than the other.
In the case of the shādh and the maḥfūẓ, both transmitters are reliable, but one is stronger than the other.
Accordingly, the shādh and the munkar are deemed outweighed, whereas the maḥfūẓ and the maʿrūf are accorded preference.

Another definition of the shādh

Some scholars did not require, in defining either the shādh or the munkar, the additional condition of contradicting another narrator—whether that other narrator be strong or weak. They held that a shādh report is any narration related by a trustworthy transmitter in which he is the sole authority, with no parallel source or corroborating support.