From ISOGG Wiki
A modal haplotype is the most commonly occurring haplotype (a set of STR marker values) derived from the DNA test results of a specific group of people. The modal haplotype does not necessarily correspond with the ancestral haplotype - the haplotype of the most recent common ancestor.
The two most commonly discussed modal haplotypes are the Atlantic Modal Haplotype, the most common haplotype in parts of Europe, associated with Haplogroup R1b, and the Cohen Modal Haplotype, the haplotype associated with the Jewish Cohanim tradition. However, a specific modal haplotype may be determined for any DNA test-based surname project or other test group.