Corpus was originally a Latin word. Its basic meaning is 'body'. The plural of corpus is corpora: see -us in Latin. Although some people say corpuses, this is frowned on - at least by academics who like to pretend that they speak Latin. For an etymological disquisition on corpus and some similar words, you may want to see Corps - corpse - corpus - corse.
Outside Law and Anatomy, where it is usually part of a Latin phrase (e.g. corpus delicti, "a collective name for the sum or aggregate of the various ingredients which make a given fact a breach of a given law" (Austin, John, Lectures on Jurisprudence (1879) I. xxiv. 479); "also, in lay use, the concrete evidence of a crime, esp. the body of a murdered person" (OED, 1893) - the latter, common, usage, is doubtless based on confusing 'corpus' with 'corpse' - and corpus callosum (the 'tough body' of white tissue that connects the right and left hemispheres of the brain) respectively), in modern English corpus is mostly used to mean 'a body, or collection, of writing or texts'. We can talk of 'the corpus of the Civil Law', meaning all the written basis of the Civil Law, or 'the corpus of Shakespeare's writings', meaning the Complete Works; and dictionaries are increasingly based on corpora of texts collected from language in use. Indeed, there is a branch of linguistics called 'corpus linguistics'.
Corpus belonged to a smallish set of Latin nouns ending in -us that formed plurals in -ora. (Most nouns that ended in -us formed plurals in -i.)
- Corpus is used as an informal name for the Colleges of Corpus Christi in both Oxford University and Cambridge University. This reflects the Christian (mostly Roman Catholic veneration of Corpus Christi, 'the body of Christ.