A liquid is a phase of matter between the crystalline solid state and the gaseous state.  The large scale crystalline order of the true solid is absent but the liquid has a small scale close relationship between adjacent atoms.  There is no definitive theory of the liquid state.  It is considered that the molecules comprising liquids are attracted to each other by low energy bonds types such as the Van der Waals Bond.  These bonds being easily broken by heat when vaporisation is taking place.

A liquid has a fixed volume but it takes the shape of the containing vessel.   It normally has a significantly greater density than the gaseous state but its density is generally not greatly different to the solid state.   The volume of a liquid is not significantly affected by changes in temperature or pressure

A liquid has an important different characteristic to the gas in having a free surface which provides the upper boundary on the volumes..

The physical (as opposed to the chemical ) properties of liquids are important to engineers as

On examining the periodic table the majority of elements are solid at STP.   The only liquids are Br (Bromine) and Hg (mercury).  The only gases are H(Hydrogen), He (Helium), N(Nitrogen), O(Oxygen) F(flourine),Ne (Neon) Cl(chlorine) Ar (Argon), Kr(Krypron), Xe(Xenon), Ra (Radon).

The properties of various representative solids are provided on page Liquid properties.

Thermodynamic and heat transfer relationships for fluids can be found on page Thermodynamics /Heat Transfer

Fluid flow notes can be found on page Fluids
Fluid viscosity notes can be found on page .Viscosity Notes
Fluid lubrication notes can be found on page Liquid lubrication