A neologism is a word, term, or phrase that has been recently created (or "coined"), often to apply to new concepts, to synthesize pre-existing concepts, or to make older terminology sound more contemporary. Neologisms are especially useful in identifying inventions, new phenomena, or old ideas that have taken on a new cultural context. The term e-mail, as used today, is an example of a neologism.

The etymology of neologism is rooted in the Greek language: the modern form is based on νεολογισμός [neologismos] (from νέος [neos] new + λόγος [logos] word, speech, discourse + suffix -ισμός [-ismos] -ism) - literally, "new word".

Neologisms are by definition "new", and as such are often directly attributable to a specific individual, publication, period or event. The term "neologism" was itself coined around 1800, so in the early 19th century, the word "neologism" was itself a neologism.

In psychiatry, the term is used to describe the use of words that only have meaning to the person who uses them, independent of their common meaning. It is considered normal in children, but a symptom of thought disorder (indicative of a psychotic mental illness, such as schizophrenia) in adults. Use of neologisms may also be related to aphasia acquired after brain damage resulting from a stroke or head injury. People with autism may also create neologisms.

In theology, a neologism is a relatively new doctrine (for example, rationalism). In this sense, a neologist is an innovator in the area of a doctrine or belief system, and is often considered heretical or subversive by the mainstream clergy or religious institution(s).