  Aims. We describe the photometric content of the second data release of the
Gaia project (Gaia DR2) and its validation along with the quality of the data.
Methods. The validation was mainly carried out using an internal analysis of
the photometry. External comparisons were also made, but were limited by the
precision and systematics that may be present in the external catalogues used.
Results. In addition to the photometric quality assessment, we present the best
estimates of the three photometric passbands. Various colour-colour
transformations are also derived to enable the users to convert between the
Gaia and commonly used passbands. Conclusions. The internal analysis of the
data shows that the photometric calibrations can reach a precision as low as 2
mmag on individual CCD measurements. Other tests show that systematic effects
are present in the data at the 10 mmag level.
