The mediopassive voice is a grammatical voice which subsumes the meanings of both the middle voice and the passive voice.
Languages of the Indo-European family (and many others) typically have two or three voices of the three: active, middle, and passive. "Mediopassive" may be used to describe a category that covers both the middle (or "medium") and the passive voice. In synchronic grammars, the mediopassive voice is often simply termed either "middle" (typical for grammars of e.g. Ancient Greek) or "passive" (typical for grammars of e. g. modern Danish).
In the oldest Indo-European languages, the distinction active/middle was the most important, while the development in later languages has generally been to replace the old distinction with (or reinterpret it as) an active/passive distinction (e.g. modern English: to tease / to be teased). The Proto-Indo-European language itself is typically reconstructed as having two voices, active and mediopassive, where the middle-voice element in the mediopassive voice was dominant. Ancient Greek also had a mediopassive in the present, imperfect, perfect, and pluperfect tenses, but in the aorist and future tenses the mediopassive voice was replaced by two voices, one middle and one passive. Modern Greek and Albanian have only mediopassive in all tenses.
Nevertheless, in some languages the middle or mediopassive still shows some presence, as in the examples below from two separate branches of the Indo-European languages (in Spanish, which is a Romance language, with periphrastic forms, and in Scandinavian with fully inflected forms). Significantly, the forms in both Spanish and Scandinavian are historically derived from periphrastic constructions involving the verb and the reflexive pronoun, so that the development seems to be similar to the ancient shift from an original (predominantly) medial voice to a later (predominantly) passive one.
The mediopassive can have many meanings depending on the context of the sentence.
These are a few examples of mediopassive constructions in English: The book reads well. The trousers wash easily. Ripe oranges peel well. This form is hardly ever used in English with the active voice or passive.
Spanish is an example of a modern language with a mediopassive voice, normally indicated by the use of a reflexive pronoun. This can variously have a middle-voice meaning (subject acting onto itself, or for its own benefit) or a passive-voice meaning (something acts onto the subject). An example sentence is El padre se enojó cuando su hijo rompió la lámpara. The English translation is "The father got angry when his son broke the lamp." The verb se enojó is said to be mediopassive because it comprises the reflexive pronoun se and the simple verb enojó, which together literally mean "angered himself." This would be literally translated "The father angered himself when his son broke the lamp." Pragmatics quickly rejects the middle-voice meaning for the intended mediopassive-voice meaning, translated above as "got angry," because the mediopassive voice is rarely used. Some other Spanish verbs that exhibit this property are molestarse (to be bothered) and entristecerse (to get sad).
The examples below are from Danish, but the situation is the same in Swedish and Norwegian. The passive use of the Scandinavian mediopassive is probably predominant, but the medial use is quite frequent as well. Here are examples of sub-categories of the middle voice.
Classical Armenian had a mediopassive form which was marked by changing the verb's thematic vowel instead of with a unique conjugation like in other Indo-European languages. Modern Armenian has retained some of these active/mediopassive pairs, but the distinction between the two voices is no longer productive.