In the strictest sense, I think "middle voice" means something that is
in the middle between active and passive.
active: I dress someone
passive: I am dressed by someone
middle: I dress, I get dressed, I dress myself
In the active, the subject acts on someone or something else. In the
passive the subject is acted upon. In the middle, the subject both
acts and is affected by the action, or else it's impossible to draw a
clear line between action and experience/result (e.g. with mental
activity). In English we often just use the same form as the active
where other European languages use reflexive forms.
Old Norse has a reflexive pronoun, accusative 'sik', dative 'sér',
genitive 'sÃn', which was used both with a passive and a middle
meaning. By the time the language came to be written down
extensively, a verbal suffix had developed out of the
accusative 'sik'. This was contracted to -sk and used for most
persons, although -mk appears in the first person, from 'mik'. But by
a regular sound change, when -s- comes after 't' or 'ð' it
becomes 'z', hence 'býðr' + sk > býzk "offer oneself". But 'býr' + sk
> 'býsk' "prepare (oneself), get ready".
There seems to be some freedom about when to use the seperate pronoun,
or the suffixed form: klæða sik, klæðask, both mean "to dress
oneself". You might find books referring to "the middle voice", or
else just "reflexives", "reflexive verbs", "reflexive forms", or
even "mediopassive"--all meaning these same suffixed verbs.
"middle voice" is a bit of an abritrary name though, because although
these verbs with the reflexive suffix do sometimes have a middle
meaning in that strict sense, in fact, in Old Norse, they often have
purely passive meanings, or a reciprocal meaning, e.g. berjask "fight
(one another)", or even meanings which might seem active to us. This
last category includes a lot of verbs to do with emotions: óttask,
hræðask "be(come) afraid", reiðask "get angry", dásk "admire", or
which might imply an emotional state: heitask "threaten", and verbs to
do with mental activity: minnask "remember". These are sometimes
called DEPONENT verbs (which Patricia will know all about from Latin:
verbs that have a passive form, but correspond to active verbs in
English), but it's not always easy distinguish between passive-middle-
deponent, etc. So, what the "middle voice" means in practice depends
a lot on the individual verb.
Sometimes the middle voice can be thought of as making a transitive
verb intransitive: bregða "cause something to move quickly, draw (a
weapon), thrust" : bregðask við "make a sudden movement with one's
body, jerk, react".
Just a few thoughts there. Hope I haven't added to the confusion!