Bus compression
Gentle compression on a group or the whole mix bus, so the instruments move as one rather than each on its own.
The settings are deliberately timid: a low ratio such as 2:1 or 4:1, a slow attack that lets drum hits through, auto or fast release, and rarely more than a couple of dB of gain reduction. The aim is not to be noticed but to make things that play together breathe together. Engineers call it "glue", and nobody has managed a better definition.
The template is the centre-section compressor on SSL's 4000-series consoles from the 1980s, a VCA design that sat across practically every major pop mix for a decade and has been cloned in hardware and plugins ever since. The risk is the usual one with compression: overdo it in the mix and there is nothing left for mastering to work with.