Definition of "fluid"

An extreme version of vorticity is a vortex. The vortex is a spinning, cyclonic mass of fluid, which can be observed in the rotation of water going down a drain, as well as in smoke rings, tornados and hurricanes.

2013 March, Frank Fish, George Lauder, “Not Just Going with the Flow”, in American Scientist‎, volume 101, number 2, archived from the original on 1 May 2013, page 114

fluid inclusion Petrology, a tiny fluid- or gas-filled cavity in an igneous rock. 1-100 micrometers in diameter, formed by the entrapment of a fluid, typically that from which the rock crystallized.

1992, Christopher G. Morris, Academic Press, Christopher W. Morris, Academic Press Dictionary of Science and Technology, Gulf Professional Publishing, page 854

The Doctor: Get a good night's sleep and drink plenty of fluids. / Kes: Fluids? / The Doctor: Everybody should drink plenty of fluids.

1995, David Kemper, Michael Piller, “Time and Again”, in Star Trek: Voyager, season 1, episode 4, spoken by The Doctor and Kes (Robert Picardo and Jennifer Lien)

For studying interfaces between solid and another solid, fluid, or gas, a sample can be oriented with its reflecting surface(s) vertical (and with the scattering plane, as defined by nominal incident and reflected wavevectors, horizontal).

2006, Jörg Fitter, Thomas Gutberlet, Neutron Scattering in Biology: Techniques and Applications, Springer Science & Business Media, page 236

Tenderness: is the lump tender?Composition: is the mass solid, fluid or gas?

2011, Andrew T Raftery, Michael S. Delbridge, Marcus J. D. Wagstaff, Churchill's Pocketbook of Surgery, International Edition E-Book, Elsevier Health Sciences, page 11

The choke manifold then expels the fluid or gas to the gas buster or a panic line. The panic line will then either send the fluid or gas to the reserve pit or a flare stack or flare tank.

2012, Will Pettijohn P.E.C., Oil & Gas Handbook: A Roughneck's guide to the Universe, AuthorHouse, page 23

Economics is a messy discipline: too fluid to be a science, too rigorous to be an art. Perhaps it is fitting that economists’ most-used metric, gross domestic product (GDP), is a tangle too. GDP measures the total value of output in an economic territory. Its apparent simplicity explains why it is scrutinised down to tenths of a percentage point every month.

2013 August 3, “Boundary problems”, in The Economist, volume 408, number 8847

“Oh, Loki made sure of that. My mortal parents blamed him for the way I was, for being fluid.”

2017, Rick Riordan, Magnus Chase and the Hammer of Thor , page 274 (the genderfluid character Alex Fierro is speaking)