When talking about tuners, what does 'chromatic' mean?

This refers to how the tuner operates. A chromatic tuner is able to identify all of the notes in the chromatic scale (in other words, every note in Western music) and name the pitch your plucked string is closest to. Some tuners can only identify the standard six notes (EADGBE) of the guitar, meaning that you cannot use the tuner for downtuning or using altered open tunings and so on. A chromatic tuner will recognise and work with all of these pitches, making it an essential choice for those looking to retune and experiment with their guitar tunings.