A synthetic language is not the result of a chemical process involving words and various substances. It is a language that contains a large number of morphemes per word. Morphemes are the smallest units of meaning within a word. The opposite of a synthetic language is an ‘isolating language’ that has a small number of morphemes per word. Languages with a high ratio of morphemes per word include languages like Hungarian and Finnish that are prone to long conjugations.
Words are comprised of morphemes. For example, the word ‘etymologically’ is comprised of four morphemes. These morphemes are etym-ologi-cal-ly. Just because a word is long does not mean that it has many morphemes. The word ‘conjugation’ has just two morphemes: conjugat-ion. Short words can be multi-morphemes like ‘cats,’ which has a plural or a single morpheme like ‘break.’
There are a number of reasons for certain languages having more morphemes than others. For example, English adds an extra morpheme that Japanese does not because English pluralizes nouns. Germanic languages are prone to high numbers of morphemes because they produce a number of compound words such as ‘Schwarzwaldkirschetorte,’ which means ‘Black Forest cherry gateau.’
Another cause of high morpheme ratios is conjugation. Whereas English uses a large number of pronouns and articles to give words meaning, other languages such as Hungarian conjugate their words by adding morphemes. Such languages, including Latin, also add a wider range of morphemes such as dative and accusative morphemes.
There are, of course, different degrees of synthesis in a synthetic language and between synthetic languages. English is, relatively speaking, towards the isolating end of linguistics, but sentences can be formed with high or low levels of synthesis depending on how the meaning is expressed. Chinese and Japanese are isolating languages.
On the other extreme, languages such as Finnish have a high level of synthesis. This allows the language to express more information using fewer words, but also means the words are longer and more difficult to pronounce. Languages cannot be judged on a single sentence, as it is possible in many languages to frame the sentence in such a manner as to give it more or less of an appearance of being a synthetic language.
A highly synthetic language, such as Mohawk, may also be determined as a polysynthetic language. This means that words can have a high number of morphemes on a regular basis and sometimes entire sentences can be expressed using a single word. Other examples of polysynthetic languages include Ainu in Japan and Chukchi.