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Chemotactic drift is the directed velocity associated with sensing a chemical gradient. In a simple continuum model the drift is $\chi \nabla c$; multiplying by cell density gives the chemotactic flux.

A chemotactic flux is the transport flux caused by directed cell movement in response to a chemical gradient. For a cell density $q$ and attractant concentration $c$, a common flux is $\chi q\nabla c$ with $\chi >0$.