Zero plural marker nouns
A zero plural marker is a noun that appears singular but is actually a plural.
Think of a noun, any noun. Because I’m me, I instantly think cat, so let’s go with that for a moment. Cat is what is referred to as a count noun, a noun that can be counted. When there are multiple, the word cat becomes cats. The ‘s’ here is called a plural marker. When words abide by this rule (or other variations of the rule—think candelabrum and candelabra, for example), they are regular nouns.
However, when a noun that becomes plural does not include the plural marker, it is called a zero plural marker noun. They are irregular nouns and are few in number. Here are some examples:
One sheep, two sheep, many sheep
One deer, two deer, many deer
One fish, two fish, many fish
One aircraft, many aircraft
One offspring, several offspring
A single moose, too many moose
The bison, the herd of bison
One foot, six foot (measurement)
A pound, ten pound (currency)
A stone, three stone (weight)
However, do not confuse a zero plural marker noun with a mass noun, as explained here.