Szlachta
Today the word szlachta in the Polish language simply translates to "nobility". In its broadest meaning, it can also denote some non-hereditary honorary knighthoods granted today by some European monarchs. Occasionally, 19th-century non-noble landowners were referred to as szlachta by courtesy or error, when they owned manorial estates though they were not noble by birth. In the narrow sense, szlachta denotes the old-Commonwealth nobility.

Q: Why were the non nobles and nobles referred as the same term?
by courtesy or error