A specific, pre-fabricated sequence of DNA or RNA, labeled by one of several
methods, used to detect the presence of a complementary sequence by binding
(hybridizing) to that site.
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
A probe is a single-stranded sequence of DNA or RNA used to search for its complementary sequence in a sample genome. The probe is placed into contact with the sample under conditions that allow the probe sequence to hybridize with its complementary sequence. The probe is labeled with a radioactive or chemical tag that allows its binding to be visualized. In a similar way, labeled antibodies are used to probe a sample for the presence of a specific protein.