Internalism and externalism
Internalism has a variety of different meanings within philosophy, and for each of these meanings there is a corresponding externalist position which is just the denial of the internalist one. There are different internalist positions in semantics, philosophy of mind, epistemology and ethics.
Within epistemology, internalism is normally understood as a position about the concept of justification, although similar claims are sometimes also made about knowledge. There are a variety of such positions, described below. It should be noted that theories such as reliabilism are in conflict with most versions of epistemological externalism.
Accessibilism is the position that whether someone's belief is justified supervenes only on facts to which that person has some sort of access. There are a variety of different views as to what form this access must take, and so a variety of different forms of accessibilsm. The most common position is that it need only involve reflection.
Philosophy of mind
Within philosophy of mind, internalism is the theory that the content of one's mental states does not depend on the external world.
Semantic externalism comes in two varieties, depending on whether meaning is construed cognitively or linguistically. On a cognitive construal, externalism is the thesis that what concepts (or contents) are available to a thinker is determined by their environment, or their relation to their environment. On a linguistic construal, externalism is the thesis that the meaning of a word is environmentally determined. Likewise, one can construe semantic internalism in two ways, as a denial of either of these two theses.
Externalism and internalism in semantics is closely tied to the distinction in philosophy of mind concerning mental content, since the contents of one's intentional mental states are usually taken to be semantic objects that are truth-evaluable.
In moral philosophy, motivational internalism is the view that moral beliefs or judgments are intrinsically motivating. That is, the motivational internalist believes that there is one can only believe that something ought to be done if one is motivated to do it.