An eunuch is a man castrated for a social purpose, typically employed in royal courts working with the harem. Eunuchs are seen throughout the Old Testament and within the the Book of Acts as male workers in royal households. Eunuchs are also a subject of Jesus' teaching on Divorce and are used as figurative images twice in the Book of Isaiah.
Hegai and Esther
The Ethiopian Eunuch
In Isaiah, the plight of eunuchs is considered among other peoples who ordinarily would have been rejected in Israel.
When questioned by the Pharisees about legitimate reasons for divorce, Jesus condemned any divorce outside of sexual immorality. The disciples, believing this to be unreasonable expressed that if marriage were to be such a strict bond that it would be better to never marry.
Jesus' response was that only certain people could chose to accept a life without marriage, mentioning three kinds of "eunuchs" as those who could accept it. The first group was natural born "eunuchs", or those who were born with a biological deformity (thus choosing to live celibate out of inability or lack of desire for sexual relations). The second group was those who had castrated, referring to literal eunuchs. Like the former, they too would normally accept a life celibacy out of the inability to have sexual relations. Lastly, Jesus talked of those who have "castrated" themselves for the Kingdom of Heaven. Figuratively speaking, Jesus was referring to those who voluntarily chose to have celibate lifestyles out of some greater purpose in worshiping God. In the same way that the two previous groups would never have sexual relations, so those who chose this lifestyle would never have such relations; in this sense they were "castrated", as if they were unable to have those relations physically.