Exposure and shooting
Push processing
Rating a film faster than its box speed and developing it for longer to make up for the underexposure.
You meter as if the film were faster — for example, as if it had one stop more speed — and extend the development time. Pushing usually raises contrast and grain and can lose shadow detail, so it works best when the light is poor and you need the speed. The film and developer datasheets are the authority on how far a given film can be pushed; the times on this site say which speed each was published for.