Mean diffusivity (MD) is a metric derived from diffusion-weighted magnetic resonance imaging (DW-MRI) data. It quantifies the average magnitude of water molecule diffusion within a particular tissue or region of interest. MD is calculated based on the diffusion tensor, which characterizes the rate and directionality of water diffusion in tissue. Unlike fractional anisotropy (FA), which measures the degree of diffusion directionality, MD represents the overall magnitude of diffusion regardless of direction.