a solid solution

You know how you can stir sugar into water and it vanishes into a single clear liquid? That is a solution: one substance dissolved evenly in another, mixed all the way down to the molecular scale. A solid solution is the same idea, but frozen solid: foreign atoms (the solute) are dispersed uniformly throughout the host crystal (the solvent), and the whole thing stays a single, homogeneous crystalline phase. Nothing separates out; there is no visible boundary between host and guest.

The crucial point is that the host keeps its crystal structure. When you add nickel to copper, the nickel atoms simply take some of the copper positions in the same FCC lattice — you still have one FCC crystal, just with two kinds of atom sharing it. There are exactly two ways for the guest atoms to fit: they can replace host atoms on the regular lattice sites (a substitutional solid solution), or, if they are small enough, they can tuck into the gaps between host atoms (an interstitial solid solution). Brass (copper with zinc), sterling silver, and stainless steel's chromium-in-iron are all solid solutions.

How much you have of each is the composition, and there are two honest ways to state it. Weight percent (wt%) is the mass of one element divided by the total mass — what a metallurgist weighs out. Atom percent (at%) is the count of atoms of one element divided by the total atom count — what physically matters for how the lattice fills. They differ whenever the elements have different atomic masses, so always check which one a number means. Solid solutions matter because dissolving even a little of the right element can dramatically strengthen a metal (solid-solution strengthening) without turning it brittle.