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This article doesn't say anywhere. —Preceding unsigned comment added by 24.13.162.248 ( talk) 05:17, 5 October 2010 (UTC) reply

It's the electric potential; the first equation defines the electric field as ${\displaystyle E=-\nabla \phi -{\frac {\partial \mathbf {A} }{\partial t}}}$ where, according to "standard usage" ${\displaystyle \phi }$ is the electric potential and ${\displaystyle \mathbf {A} }$ is the magnetic (vector) potential -- Osquar F ( talk) 08:12, 15 August 2012 (UTC) reply