Gregor Mendel was an Austrian monk who lived from 1822 to 1884, known for their experiments with pea plants between 1856 and 1863, which helped establish the rules of heredity. He is sometimes referred to as the father of genetics.
Back in his time, genes were not named like that, in fact, some people thought hereditary worked as mixing liquids, so that the characteristics would mix or dilute over time. Mendel believed there were heredity units, that did not mix like liquids, he called them "factors", and they are what we now call genes.
So, the answer would be that Mendel used the term "factors" to refer to "genes".