A quantitative parameter derived from diffusion tensor imaging that measures water molecule diffusion parallel to the principal direction of the diffusion tensor. Axial diffusivity (AD or AxD) represents the largest eigenvalue (λ1) of the diffusion tensor and provides information about axonal integrity and tissue microstructure. This measurement can be applied to various biological tissues including brain white matter, skeletal muscle, kidney, and other organs. The trait is Experimental Factor Ontology id EFO_0920013 (axial diffusivity measurement). Also known as: axial diffusivity, longitudinal diffusivity, parallel diffusivity.