Solid describes the state of matter in which atoms are most tightly packed.

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The main idea here is how closely the particles fit together in different states of matter. In a solid, the atoms are arranged in a fixed, tightly packed lattice, so they stay in place and the substance has a definite shape and volume. The particles vibrate a little but don’t flow past each other, which is why solids feel rigid and dense.

In liquids, the particles are less tightly bound and can slide past one another, so liquids keep a definite volume but take the shape of their container. Gases have particles that are far apart and move freely, filling all available space and having much lower density. Plasma is a high-energy state where atoms are ionized, so the particles aren’t the same tightly packed arrangement as in a solid.

So, because solids have the most compact, orderly arrangement of particles, they are the state described as the most tightly packed.