What is a Neighbor?:
Typically, the concept of neighbor is limited to those people one lives near, or at least people in the local community. This is how the Old Testament sometimes uses the term, but it is also used in a broader or figurative sense to refer to all Israelites. Thus the commands not to covet a neighbors wife or possessions refers to all fellow Israelites, not just those who happen to live in the vicinity.
Neighbors in the Old Testament:
The Hebrew word most often translated as neighbor is rea and has a variety of connotations: friend, lover, and of course the usual sense of neighbor. In general, it might be used to refer to anyone who isnt an immediate kinsman or an enemy. Legally, it was used to refer to any fellow member of the covenant with God, in other words fellow Israelites.
Neighbors in the New Testament:
One of the best remembered of Jesus parables is that of the Good Samaritan who stops to help an injured man when no one else would. Less well remembered is the fact that this parable was told to answer the question Who is my neighbor? Jesus answer suggests the broadest possible interpretation for neighbor, such that it even includes members of unfriendly tribal groups. This would be consistent with his command to love ones enemies.
Neighbors and Ethics:
Identifying who ones neighbor is has occupied a great deal of discussion in Jewish and Christian theology. The broad use of neighbor in the Bible appears to be part of a general trend through the entire history of ethics, which is to increasingly broaden the social circle of ones ethical concern. Noteworthy is the fact that its always used in the singular, neighbor, rather than the plural this highlights ones ethical duty in particular cases to specific people, not in the abstract.