A Segment of DNA is a defined sequence of base-pairs from a given Chromosome or Mitochondria strand.  Usually defined with a specific start and end point on the strand.  Often a count in Build37 nomenclature.  A Gene is a segment of DNA that is known to have an important, biological function.  Sometimes a Segment may vary in actual length (slightly, as measured by base-pairs) within two individuals because of STR or SNP value differences.  The Build37 nomenclature takes this into account. Due to these differences, and tips of chromosomes being volatile over time, defining a segment by base-pair counts is somewhat difficult. SNPs and STRs are often defined with a location based on the Build37 standard or similar.

In Genetic Genealogy Testing, when looking for a matching segment, the segment is often defined by a sequence of tested SNP values.  The location of each SNP is known and so the segment length is calculated using the two outermost or end-point SNP’s of the segment.  Often, the exact position or length will then be rounded; like to the nearest 0.1 centiMorgan (cM). The centiMorgan is a usual measure of the segment length but is not an actual count of the base pairs.

In Autosomal matching in Genetic Genealogy Testing, the total matching segment length is the sum of all the matching segment lengths on the Autosomes and sometimes includes the X matches also. The longest matching segment (length) is simply the greatest length matching segment in the autosomes (or possibly separately specified for the xDNA). The number of matching segments is the count of all the matching segments on the autosomes; usually above a certain minimum segment length. The number of matching chromosomes is the count of all chromosomes containing at least one matching segment.