Portmanteau

A portmanteau is used broadly to mean a blend of two (or more) words, and narrowly in some linguistics fields to mean only a blend of two or more function words.

Meaning
"Portmanteau word" is used to describe a linguistic blend, namely "a word formed by blending sounds from two or more distinct words and combining their meanings".

Such a definition of "portmanteau word" overlaps with the grammatical term contraction, and linguists avoid using the former term in such cases. As an example: the words do + not become the contraction don't, a single word that represents the meaning of the combined words.

Origin
The usage of the word 'portmanteau' in this sense first appeared in Lewis Carroll's book Through the Looking-Glass (1871), in which Humpty Dumpty explains to Alice the coinage of the unusual words in Jabberwocky:

"‘slithy’ means ‘lithe and slimy’... You see it's like a portmanteau—there are two meanings packed up into one word"
"‘Mimsy’ is ‘flimsy and miserable’ (there's another portmanteau ... for you)".

Carroll uses the word again when discussing lexical selection:

Humpty Dumpty's theory, of two meanings packed into one word like a portmanteau, seems to me the right explanation for all. For instance, take the two words "fuming" and "furious." Make up your mind that you will say both words ... you will say "fumious.".

Carroll suggests here a double metaphor. The original meaning of the word 'portmanteau' is a form of suitcase containing two separated hinged compartments; thus: two distinct words, packed as one. The word 'portmanteau' is itself a 'portmanteau word', deriving from the French compound "portemanteau" consisting of the conjugated word porter (to carry) and the word manteau (coat), meaning a coat hanger.

Examples
Many neologisms are examples of blends, but many blends have become part of the lexicon. In Punch in 1896, the word brunch (breakfast + lunch) was introduced as a "portmanteau word". In 1964, the newly independent African republic of Tanganyika and Zanzibar chose the portmanteau word Tanzania as its name. A spork is an eating utensil that is a combination of a spoon and fork.

"Wikipedia" is itself an example of a portmanteau word because it combines the word "wiki" and "encyclopedia". Sysop is an example as well, combining "system" and "operator."

Blaxploitation is a film genre/style, whose name derives from a portmanteau of "black" and "exploitation", reflecting its main theme of social problems and crime amongst African American people.

Portmanteau words may be produced by joining together proper nouns with common nouns, such as "gerrymandering", which refers to the scheme of Massachusetts Governor Elbridge Gerry for politically contrived redistricting: one of the districts created had the semblance of a salamander in outline. Two proper names can also be used in creating a portmanteau word in reference to the partnership between people, especially in a case where both persons are well known, or sometimes to produce epithets such as "Billary" (referring to former United States president Bill Clinton and Hillary Rodham Clinton) or TomKat (referring to Tom Cruise and Katie Holmes). In this example of recent American political history, the purpose for blending is not so much to combine the meanings of the source words but "to suggest a resemblance of one named person to the other" and the effect is often derogatory, as linguist Benjamin Zimmer notes.[ Portmanteaux (or portmanteaus) can also be created by attaching a prefix or suffix from one word to give that association to other words. Subsequent to the Watergate Scandal, it became popular to attach the suffix "-gate" to other words to describe contemporary scandals, e.g. "Filegate" for the White House FBI files controversy. Likewise, the suffix "-holism" or "-holic", taken from the word "alcoholism" or "alcoholic", can be added to a noun, creating a word that describes an addiction to that noun. Chocoholic, for example, means a person who is addicted to chocolate.

Portmanteau words can be used to describe bilingual speakers who use words from both languages while speaking. For instance a person would be considered speaking "Spanglish" if they are using both Spanish and English words to voice a complete thought.

Source: http://en.wikipedia.org/wiki/Portmanteau