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The adjective nominal (ultimately from Latin nōmen, "name") generally relates to the concept of names, and often to the difference between what something is in name (ideally or theoretically) and what it is in reality.

A nominal quantity (e.g., length, diameter, speed, volume, voltage, value) is generally the quantity according to which some item has been named or is generally referred to. Such a nominal value may often be different from the actual value, having a relation to the actual value that is anywhere on a spectrum from closely related (for example, differing only within a range of acceptable random variation]) to only distantly related. An example of the former is a type of battery that has an actual voltage of 1.62 V, but is called a "1.5 volt battery"; it has a nominal voltage of 1.5 V (which cannot be measured anywhere). An example of the latter is the nominal size 2×4 for dimensional lumber: its actual size is 1½″ × 3½″ (38×89 mm) (plus or minus some small tolerance).

In human cognition, there are at least two concepts that relate to a difference between what something is in name and what it is in reality: (1) whether we even perceive that the difference exists; and (2) if so, whether we judge that the difference matters to us. A judgment in one case that the difference is very important and another judgment in another case that the difference is very unimportant can both relate to the concept of "in name only": we emphasize that the difference exists, in order to implicitly emphasize either its importance or unimportance. In the latter case (unimportance), the word nominal can approach synonymity with "small" or "unimportant", such as in the phrase paying a nominal fee. The fee is a monetary expense in name only; in reality, the amount does not matter very much. In the former case (importance), the word nominal can emphasize the irony of false appearances or assumptions, such as when a certain police department is especially corrupt—because we archetypically equate police to "the good guys", we may emphasize the badness of this department by saying that they are nominally the face of law and order in their community: that is, in name only.

• In economics a nominal value indicates the listed value of an item in a monetary currency as opposed to the 'real' value in terms of purchasing power. See real versus nominal value. A nominal interest rate may refer to either a rate of interest that is not re-stated to correct for compounding, or an interest rate that does not take into account inflation (to derive the real interest rate).
• In engineering, nominal regarding measurements generally can be defined as "what is predicted without considering what deviation may occur or whether it is acceptable"; or "what is predicted plus or minus X number of standard deviations". The use of the phrase "all systems nominal" at NASA indicates that telemetry is reading as expected from historical data trends. This concept often overlaps with the layperson's sense of the word normal, but nominal does not always mean normal.
• In linguistics, a "nominal" is a word or group of words functioning as a noun. The word is also sometimes used as a shortened form of "nominal phrase", a synonym for "noun phrase". "Nominal" can also mean a sequence of one or more nouns that do not form a complete noun phrase. In other uses, it refers to a class of word that encapsulates both parts of speech in English as nouns and adjectives.
• In statistics, nominal data is a form of categorical data where the order of the categories is not significant. This is sometimes contracted to nominals.
• In law, nominal damages may be assessed by a jury or judge in a lawsuit when it is found that the defendant committed the act complained of by the plaintiff, but that no harm was suffered by the plaintiff as a result.

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