From ISOGG Wiki
A megabase is a term used in genetics to measure the length (number of base pairs) of a genome segment. Megabases measure the physical distance of a genome region whereas a centiMorgan is used to measure the genetic distance.
1 Mb = 1,000,000 bases = 1 Megabase
deCODE genetics use megabases to measure matching chromosome blocks among their customers. AncestryDNA originally measured matches in megabases but converted to centiMorgans in December 2013. For a technical explanation of the megabase-based matching system previously used by AncestryDNA see the blog post Ken Chahine answers my questions by CeCe Moore.