Incorporation is a phenomenon by which a grammatical category, such as a verb, forms a compound with its direct object (object incorporation) or adverbial modifier, while retaining its original syntactic function.
Incorporation is central to many polysynthetic languages such as those found in North America, Siberia and northern Australia. However, polysynthesis does not necessarily imply incorporation (Mithun 2009); neither does the presence of incorporation in a language imply that that language is polysynthetic.
Although not used regularly, English shows some instrument incorporation, as in breastfeed, and direct object incorporation, as in babysit. Etymologically, such verbs in English are usually back-formations: the verbs breastfeed and babysit are formed from the adjective breast-fed and the noun babysitter respectively. Incorporation and plain compounding may be fuzzy categories: consider backstabbing, name-calling, axe murder.
The following example from Oneida (Iroquoian) illustrates noun incorporation.
In this example, the verbal root hninu appears with its usual verbal morphology: a factive marker (FACT), which very roughly translates as past tense, although this is not quite accurate; an agreement marker (1.SG), which tells us that the verb agrees with 1st person singular (the speaker); and an aspect marker, punctual (PUNC), which tells us that this is a completed event. The direct object ne kanaktaʼ follows the verb. The function of the particle ne is unclear. Note that the word for bed consists of a root nakt plus a prefix and a suffix. The notion of the root is important here, but the properties of the prefix and suffix do not matter for this discussion.
The following sentence means the same as above, but noun incorporation has taken place.
In this example, the root for bed nakt has incorporated into the verbal construction and appears before the verbal root. Two other incidental changes are noticed here. First, the agreement marker in the first example is k and in the second example is ke. These are two phonologically-conditioned allomorphs. In other words, the choice between using k and ke is based on the other sounds in the word (and has nothing to do with noun incorporation). Also, there is an epenthetic vowel a between the nominal and verbal roots. This vowel is inserted to break up an illegal consonant cluster (and also has nothing to do with noun incorporation).