Recently internalism and externalism have become part of the standard jargon of philosophical discourse, and have become central to certain important debates.

Moral Philosophy

In contemporary moral philosophy, internalism is the view that moral beliefs function as a motivating factor. That is the internalist believes that there is an internal connection between her belief that "X ought to be done" and her motivation to do X. In the same way an externalist would argue that there is no essential internal connection between moral beliefs and motives. An externalist would say that there's no essential reason that the belief "X is wrong" leads to a desire not to do X. It is likely that this use of these terms comes from W.D. Falk's paper Ought and motivation (1948).

Epistemology

In contemporary epistemology the belief that everything necessary to provide justification for a belief is immediately available to the consciousness is called internalism. Externalism in this context is the view that there are factors other than those which are internal to the believer which govern whether one can be warranted in calling that belief knowledge. One strand of externalism is loosely called the Causal theory of knowledge , and reliabilism is sometimes considered to be another strand.

External link: Internalist vs. Externalist Conceptions of Epistemic Justification

Semantics

Semantic internalism comes in two flavours, depending on whether meaning is construed cognitively or linguistically. Depending on the flavour, it says that the meaning of a term is completely determined by either the contents of the mind in question, or by what language users know when they know the language in question. Semantic externalism says that meaning depends, at least in part, on something out in the world, or on the relationship between the mind and the world.

See also:

• Linguistic turn and cognitive turn for more about the two construals of meaning.
• Twin Earth thought experiment

Philosophy of Mind

Within the context of the philosophy of mind, externalism is the theory that mental states are dependent on their relationship to the external world. This is in contrast to both a Cartesian dualism which posits two separable realms for thought and extension (or to use more modern terms, mind and matter), as well as George Berkeley's radical idealism. Externalists generally claim that thoughts or other mental states are caused by external forces, but also that thought, feelings, and all of the other mental states could not exist if except as part of a larger world external to thought. Internalism in this context is precisely the opposite. They would claim that there is nothing essential to the concept of a mental state which requires the existence of an external world.