The son of someone's first cousin is their first cousin once removed. Cousins of the same generation share at least one set of grandparents and are called "first cousins." The children of first cousins are one generation away, or "once removed" from the generation.
The word "removed" in genealogy is used to indicate that two people are from different generations. For example, "once removed" indicates a difference of one generation and "twice removed" indicates a difference of two generations. Cousins given labels such as "first," "second" and "third" to indicate the relatives they share. For example, second cousins share at least one set of great-grandparents, while third cousins share a set of great-great grandparents.