  Terzan 5 is a stellar system in the Galactic bulge commonly catalogued as a
globular cluster. Through dedicated NIR photometry and spectroscopy we have
discovered that it harbors two main stellar populations defining two distinct
red clumps (RCs) in the colour-magnitude diagram, and displaying different iron
content: [Fe/H] = -0.2 and [Fe/H]=+0.3 for the faint and the bright red clumps,
respectively. In addition, a third minor population with significantly lower
metallicity ([Fe/H]=-0.79) has been recently detected, thus enlarging the
metallicity range covered by Terzan 5 to Delta[Fe/H] ~ 1 dex. This evidence
demonstrates that, similarly to omega Centauri in the Galactic halo, Terzan 5
is not a genuine globular cluster, but a stellar system that experienced a much
more complex star formation and chemical enrichment history. Moreover the
striking chemical similarity with the bulge stars suggests that Terzan 5 could
be the relic of one of the massive clumps that contributed through strong
dynamical interactions with other similar sub-structures) to the formation of
the Galactic bulge.
