DNA or RNA segment tagged with a radioactive molecule is called

A probe is a single-stranded DNA or RNA segment tagged with a radioactive molecule.
It is allowed to hybridise to its complementary DNA in a clone of cells, and the hybrid is then detected by autoradiography.
A clone having a mutated gene does not appear on the photographic film, because the probe is not complementary to the mutated gene.
Probes are also used in DNA fingerprinting (VNTR probes).
A plasmid is an extra-chromosomal circular DNA, a vector carries foreign DNA into a host and a clone is a group of identical cells or DNA copies.
Hence such a tagged segment is called a probe.