A Visual Diary is a notebook for gathering thoughts, ideas and inspiration.
It is put to use by artists of all kinds (fine, plastic, graphic, etc) to record ideas and flashes of inspiration wherever or whenever they occur.
A drawing tutor of mine shared a story of how keeping an old VD helped her find a starting point for a new project after rediscovering her fascination with crumpled money - seeing her old sketches and drawings and pictures she had acquired of it at different angles got her re-inspired her for a new project.
This form of referencing aids in recapturing creativity after a creative slump and can be as varied as the artists who keep them, some plain, some heavily ornamented, some neatly arranged and yet others haphazardly put together (my wine-loving ceramics lecturer comes to mind here). Oftentimes keeping it updated is the inspiration itself.