Liquid
A liquid is a state of matter with a definite volume but no fixed shape; its particles are close together but can slide past one another.
Definition
A liquid is a state of matter with a definite volume but no definite shape, because its particles are held close together by attractive forces yet have enough energy to slide past one another rather than lock in place. Liquids flow, take the shape of their container, and barely compress, and their behavior is described by viscosity, the resistance to flow, and surface tension, the tendency of the surface to contract, both of which increase with the strength of intermolecular forces like hydrogen bonding, dipole-dipole attraction, and dispersion forces. A liquid's vapor pressure, the pressure of vapor in equilibrium with it, rises with temperature until it matches the external pressure at the boiling point, and the liquid state exists only in the region of a phase diagram between the triple point and the critical point, where thermal energy and intermolecular attraction are roughly comparable so particles keep short-range order without fixed positions or long-range order.
Example
Honey is more viscous than water and flows slowly, water's surface tension is strong enough to let a paper clip float and water striders walk on ponds, and mercury is a liquid metal at room temperature with a melting point of $-38.8^\circ\text{C}$. Water has a surface tension of $72\text{ mN/m}$ at $25^\circ\text{C}$, among the highest of common liquids, because each molecule can form up to four hydrogen bonds, compared with $22\text{ mN/m}$ for ethanol, which has only one $OH$ group, and $18\text{ mN/m}$ for hexane, which has no hydrogen bonding at all.
Key Insight
A liquid's particles sit about as close together as a solid's, which is why a substance takes up nearly the same volume as a liquid or a solid but hundreds of times more space as a gas. Liquids are also the hardest state to model theoretically, since gases are simple because their particles rarely interact and crystals are simple because they are periodic, but a liquid is simultaneously dense and disordered, so its structure can only be described statistically rather than exactly.