What is a group of sheep called?

An excellent question about English collective nouns. A group of sheep is most commonly called a flock. This is the term you will hear almost everywhere. You might also see the word herd used, which is also correct, but flock is definitely the more specific and typical term for sheep. It is one of those interesting vocabulary points that can be a little tricky when you are learning the language. I hope this information is helpful to you