Off-key is a term often used to denote musical content that is not at the expected frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 or pitch

Pitch (music)

Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...

 period, either with respect to some absolute reference frequency, or in a ratiometric sense (i.e. through removal of exactly one degree of freedom, such as the frequency of a keynote), or pitch intervals not well-defined in the ratio of small whole numbers.

The term may also refer to a person or situation being out of step with what is considered normal or appropriate.

Explanation of on-key

The opposite of off-key is on-key or in-key, which suggests that there is a well defined keynote, or reference pitch. This does not necessarily have to be an absolute pitch but rather one that is relative for at least the duration of a song. A song

Song

In music, a song is a composition for voice or voices, performed by singing.A song may be accompanied by musical instruments, or it may be unaccompanied, as in the case of a cappella songs...

 is usually in a certain key, which is usually the note that the song ends on, and is the base frequency around which it resolves to at the end.

The base-frequency is usually called the harmonic

Harmony

In music, harmony is the use of simultaneous pitches , or chords. The study of harmony involves chords and their construction and chord progressions and the principles of connection that govern them. Harmony is often said to refer to the "vertical" aspect of music, as distinguished from melodic...

 or key

Key (music)

In music theory, the term key is used in many different and sometimes contradictory ways. A common use is to speak of music as being "in" a specific key, such as in the key of C major or in the key of F-sharp. Sometimes the terms "major" or "minor" are appended, as in the key of A minor or in the...

 center. Being on-key presumes that there is a key center frequency around which some portion of notes have well defined intervals to.

Deliberate use off-key content

Examples include the words "Thought He Was a Goner" in the song "And the Cat Came Back

And the Cat Came Back

"The Cat Came Back" is a comic song written by Harry S. Miller in 1893. "The Cat Came Back" has since entered the folk tradition and been recorded under variations of the title—"But the Cat Came Back", "And the Cat Came Back", etc...

" and the words "Yum Yum" in the children's song "Five Green and Speckled Frogs

Five Green and Speckled Frogs

Five Green and Speckled Frogs is a popular children's song and nursery rhyme sung in many daycare centers. It is also often sung to babies when they are learning how to swim, in "parent and tot" swim classes...

."