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A numerical prefix is a prefix that denotes a number, which is usually a multiplier for the thing being prefixed. Numerical prefixes are usually derived from the words for numbers in various languages, most commonly Greek and Latin, although this is not necessarily the case.

Numerical prefixes occur in five contexts:

• They occur in 19th, 20th and 21st century coinages, mainly the terms that are used in relation to or that are the names of technological innovations, such as hexadecimal and bicycle.
• They occur in constructed words such as systematic names. Systematic names use numerical prefixes derived from Greek, with one principal exception, nona-.
• They occur as prefixes to units of measure in the SI system. See SI prefixes.
• They occur as prefixes to units of computer data. See binary prefixes.
• They occur in words in the same languages as the original number word, and their respective derivatives. (Strictly speaking, some of the common citations of these occurrences are not in fact occurrences of the prefixes. For example: millennium is not formed from milli-, but is in fact derived from the same shared Latin root – mille.)

Because of the common inheritance of Greek and Latin roots across the Romance languages, the import of much of that derived vocabulary into non-Romance languages (such as into English via Norman French), and the borrowing of 19th and 20th century coinages into many languages, the same numerical prefixes occur in many languages.

Numerical prefixes are not restricted to denoting integers. Some of the SI prefixes denote negative powers of 10, i.e. division by a multiple of 10 rather than multiplication by it. Several common-use numerical prefixes denote vulgar fractions.

Words comprising non-technical numerical prefixes are usually not hyphenated. This is not an absolute rule, however, and there are exceptions. (For example: quarter-deck occurs in addition to quarterdeck.) There are no exceptions for words comprising technical numerical prefixes, though. Systematic names and words comprising SI prefixes and binary prefixes are not hyphenated, by definition.

Nonetheless, for clarity, dictionaries list numerical prefixes in hyphenated form, to distinguish the prefixes from words with the same spellings (such as duo- and duo).

Several technical numerical prefixes are not derived from words for numbers. (mega- is not derived from a number word, for example.) Similarly, some are only derived from words for numbers inasmuch as they are word play. (peta- is word play on penta-, for example. See its etymology for details.)

The root language of a numerical prefix need not be related to the root language of the word that it prefixes. Some words comprising numerical prefixes are hybrid words.

In certain classes of systematic names, there are a few other exceptions to the rule of using Greek-derived numerical prefixes. The IUPAC nomenclature of organic chemistry, for example, uses the numerical prefixes derived from Greek, except for the prefix for 9 (as mentioned) and the prefixes from 1 to 4 (meth-, eth-, prop-, and but-), which are not derived from words for numbers. These prefixes were invented by the IUPAC, deriving them from the pre-existing names for several compounds that it was intended to preserve in the new system: methane (via methyl which is in turn from the Greek word for wine), ethane (from ethyl coined by Justus von Liebig in 1834), propane (from propionic which is in turn from pro- and the Greek for word for fat), and butane (from butyl which is in turn from butyric which is in turn from the Latin word for butter).

This also includes the technical numeric prefixes used for systematic names. For tables of other technical numeric prefixes, see SI prefixes and binary prefixes.

• ^  Sometimes the prefixes are cited as though they were the original words themselves. The prefixes derived from Greek are not only in the wrong alphabet, but also differ from the actual corresponding Greek words. See the individual word etymologies for the actual number words.
• ^  The prefixes in this column are also unbound morphemes.
• ^  See Mendeleev's predicted elements for the most common use of these numerical prefixes.
• ^  Greek hexa-/hex- is often used for words for 6 even when Latin sexa-/sex- would be more etymologically appropriate because of the similarity to English “sex”.
• ^  The distinction between Latin and Greek is blurred in the case of 8. Unlike the other numbers, there was little divergence between Latin and Greek in the words for 8. Whilst octa- is primarily of Greek derivation, octo- and oct- can be considered to be derived from both Greek and Latin.

• Stephen Chrisomalis. Numerical Adjectives, Greek and Latin Number Prefixes. The Phrontistery.