Domain is the set of all possible input values is any relation. It means the output value in a function depends upon each member of domain. Domain value vary in different mathematical problems and depends upon the function for which is it solved. If we talk about cosine, then domain is the set of all possible real numbers either above the 0 value or below the 0 value, it could also be 0. While for square root, the domain value could not be less than 0, it should be minimum 0 or above 0. In other words, you can say that domain of square root is always 0 or positive value. For complex and real equations, the domain value is a subset of complex or real vector space. If we want to solve a partially differential equation for finding the value of domain, then your answer should lie within three dimensional space of Euclidean geometry.
