"Fluid tuning" refers specifically to the tuning system developed by Geoff Smith which makes a piano or a hammer dulcimer dynamically retunable. See http://www.guardian.co.uk/music/video/2009/nov/22/fluid-piano-classical-music for a video, see Geoff Smith's website.
Generalized, "fluid tuning" could mean any instrument which offers a "temporarily fixed" tuning. That is, at any one moment, the instrument offers a finite set of fixed pitches, yet it also allows the continuous change of what those pitches are. Each pitch-producing mechanism in the instrument (each "voice") then has a defined frequency range. A fluid tuning could then be defined by a set of interval ranges instead of a set of single pitches.
Defined in that way, instruments with fluid tuning also include the pitch bending thumb piano, the moveable-frets guitar, and perhaps even the claviola, as well as a number of interfaces made possible with computers.