Definition for Probe
From Biology Forums Dictionary
In biology, a probe is a fragment of DNA or RNA which is labeled in some way (often incorporating 32P or 35S), and which is used to hybridize with the nucleic acid in which you are interested. For example, if you want to quantitate the levels of alpha subunit mRNA in a preparation of pituitary RNA, you might make a radiolabeled RNA in-vitro which is complementary to the mRNA, and then use it to probe a Northern blot of the pit RNA.
A probe can be radiolabeled, or tagged with another functional group such as biotin. A probe can be cloned DNA, or might be a synthetic DNA strand. As an example of the latter, perhaps you have isolated a protein for which you wish to obtain a cDNA or genomic clone.