Perimeter and prime number are words predominantly used in Maths. Like other subjects, or fields of knowledge, Maths uses various English words in a specialised manner that you need to know in order to learn the subject.
Many of these words hold meanings similar to general usage, although you wouldn't want to confuse the maths terminology with the every day understanding of a word. One example is "face". The most common meaning of the term is that part of the body where the eyes, nose and mouth are located, but in maths, of course, a "face" is a flat surface of a 3D object. You can see how the two definitions are related, but it is important to bear the differences in mind, too.
How much mathematical vocabulary do you know? Geometrical terms include angle, line, plane, axis, circumference, radius, vertices, acute, obtuse, pi, parallel, perpendicular, etc. Vocabulary necessary for calculations includes numerator, denominator, fraction, decimal, quotient, product, square, square root, divisible, etc. Many mathematical terms come from other languages, such as Latin and Greek.
Have a go at this quiz to get you up to speed with mathematical terms and spelling.