Below are definitions of words and terms you'll see used on these pages as well as other websites and in books about using DNA in genealogy.
Haplogroup - A collection of closely related haplotypes defined by specific mutations [Single Nucleotide Polymorphism (SNP)].
Haplotype - A set of closely linked alleles (genes, markers or DNA polymorphisms) inherited as a unit. A contraction of the phrase "haploid genotype". Different combinations of polymorphisms are known as haplotypes. Collectively the results from several loci could be referred to as a haplotype. "Haplo" comes from the Greek word for "single".
Short Tandem Repeat - Areas of repetitive DNA within the genome where the repeating unit is very small, usually 1-6 nucleotides. These are generally polymorphic within a population and can be used for bone marrow transplant engraftment, forensics, identity testing, paternity testing, etc. Mutations in STR's, though rare, are common enough to be useful in distinguishing family lines in genealogical time periods. [Synonymous with microsatellite]
Single Nucleotide Polymorphism - A genetic change that is caused by substitution of a single nucleotide and thought to occur on time scales of 1000's of years. Used to define haplogroups and used in population genetics.
SNP - See "Single Nucleotide Polymorphism"
STR - See "Short Tandem Repeat"
Time to Most Recent Common Ancestor - A measure of how many generations or years that two Y-chromosomes are from a common ancestor, based on the number of mutations the two Y chromosomes differ. The more DNA markers are tested for, the more precise the number of generations/years.
TMRCA - See "Time to Most Recent Common Ancestor"
Y Chromosome - [also Y-DNA] The sex chromosome that is carried by men. [Normal human males have one X chromosome and one Y chromosome] The Y chromosome is passed from father to son over hundreds of years (and even thousands of years) with little or no change. As such, the Y-DNA is ideal for surname studies.