Diakonissa is a Greek title of honor that is used to refer to a deacon's wife.
It is derived from diakonos—the Greek word for deacon (literally, "server").
There does not currently seem to be any standard English equivalent, so most English-speaking Orthodox Christians will use the title most common in the old country churches from which their local family or parish finds its origin.
Romanian uses a derivative from the Greek term, Diaconiţa, as does Serbian, Đakonica/Ђаконица (pronounced jack-on-eet'-sa).
Other Slavic traditions generally use the same word for a deacon's wife that is used for a priest's wife: Matushka (Russian), Panimatushka (Ukrainian), etc.