In generative grammar and related frameworks, m-command is a syntactic relation between two nodes in a syntactic tree.
In government and binding theory, m-command was used to define the central syntactic relation of government.
However, it has been largely replaced by c-command in current[vague] research.
M-command is a broader relation than c-command, since a node m-commands every node that it c-commands, as well as the specifier of the phrase that it heads.
Like c-command, m-command is defined over constituency-based trees and plays no role in frameworks which adopt a different notion of syntactic structure.