Laws in terms of those instituted by governments are meant to provide for structure in terms as to how people are to function within a society. By legal definition, any particular act that violates these instituted and enforced laws or regulations is considered to be illegal. The legal definition of what constitutes something as illegal can prove to vary depending on various factors. The fact is that there will be different types of government and jurisdictions in which the legal requirements will differ.
Furthermore, this can also vary within one particular country where the legal system is divided in terms of a federal government, state government, and local and regional governments. For example, in the United States, there different legal requirements in certain aspects when comparing the federal and state government. Furthermore, the legal requirements will also vary from state to state, thus making the legal definition for what is illegal more complex.
Therefore, it can be said that the legal definition for the term "illegal" will differ depending on what part of the world is being discussed. The United States can prove to be an effective example in showing how the legal definition of "illegal" can change within one particular country.
For example, each state will have its own regulations and laws in regards to the use, ownership, and possession for firearms. In one state, the carry conceal of weapons may be allowed under certain restrictions, though in another, the carry conceal of weapons may be completely prohibited. Such different legal requirements will thus affect the legal definition as to how a particular action can be considered illegal.