King James Bible Dictionary

Literally bed-keeper or chamberlain, and not necessarily in all cases one who was mutilated, although the practice of employing such mutilated persons in Oriental courts was common (2 Kings 9:32; Esther 2:3). The law of Moses excluded them from the congregation (Deuteronomy 23:1). They were common also among the Greeks and Romans. It is said that even to-day there are some in Rome who are employed in singing soprano in the Sistine Chapel. Three classes of eunuchs are mentioned in Matthew 19:12.

General references
Matthew 19:12

Prohibited from certain privileges of the congregation
Deuteronomy 23:1; Isaiah 56:3-5

Influential court officials
Jeremiah 38:7-13; Jeremiah 52:25; Daniel 1:3

Those who voluntarily became (continent, probably) for the kingdom of heaven's sake
Matthew 19:12

Baptism of the Ethiopian
Acts 8:26-29

"The English form of the Greek word which means bed-keeper . In the strict and proper sense they were the persons who had charge of the bed-chambers in palaces and larger houses. But as the jealous and dissolute temperament of the East required this charge to be in the hands of persons who had been deprived of their virility, the word eunuch came naturally to denote persons in that condition. But as some of these rose to be confidential advisers of their royal master or mistresses, the word was occasionally employed to denote persons in such a position, without indicating anything of their proper manhood." -Abbott.

EU'NUCH, noun [Gr. a bed, and to keep.] A male of the human species castrated.

EU'NUCHATE, verb transitive To make a eunuch; to castrate.

EU'NUCHISM, noun The state of being an eunuch.