A contraction is a written form in which a number of words are combined into a new word. Usually, they form when two words are used in conjunction with high frequency, although some languages, such as French, make use of them to keep the spoken sound of a sentence flowing.
In English, contractions are usually represented with an apostrophe to replace the omitted letters that join the words together. They are often made of a pronoun followed by a form of the verb to be, such as I followed by am merging into I’m. This is related to the concept of the portmanteau, a word that combines two words directly, such as humongous as a combination of huge and enormous, or infotainment from information and entertainment. In the past, English made common use of many more contractions than in the modern age, in which portmanteaus appear to be the preferred construction.
Some common examples in modern English include can’t for cannot, isn’t for is not, it’s for it is, I’m for I am, he’s for he is, he’s for he has, haven’t for have not, I’ll for I will, and I’d for I would.
Sometimes, contractions can be ambiguous, such as in the case of He is and He was, both of which are represented by He’s, in which case, context serves to show which formation is intended. Some have their roots in historical words or spellings no longer in use, and so seem a bit strange when considered as combined words. Won’t is a good example of this — the expected combination being something more like willn’t as a contraction of will and not. In fact, historically, the construction was wynnot and later wonnot, which became contracted to wo’n’t and later the modern won’t.
Another example of this historical root for a construction can be found in the non-standard term ain’t. This word began as a contraction of the words am and not, producing amnot. This word was generalized to both are not and is not in the Cockney dialect of English. Because of its difficult pronunciation, it eventually evolved to the modern pronunciation and spelling of ain’t, and because of its association with the Cockney dialect — considered a sub-standard dialect by many in English grammar circles — it was relegated to non-standard English.
Some contractions have made their way in from older phrases, and many people don't even know that they are a combination of words. Example of this include o’clock for the phrase of the clock, fo’c’s’le for forecastle, or ne’er-do-well for never do well. Older contractions have also fallen out of common usage, though they are easily found in the work of older English authors such as Shakespeare or Milton. Examples of these include e’en for even, ’twas for it was, and o’er for over. Many of these early forms may also have served the purpose of changing commonly used words and phrases from two syllables to one, helping the author to match a set meter.