Who does god refer to as the lost sheep in the parable the lost sheep?

The parable was told because of the attitudes of two groups of Israelites - the scribes and Pharisees. The Pharisees carefully carried out the rituals of the law while the scribe were the interpreters of the law. Both of these groups were self righteous and rejected Jesus. This parable along with the parables of the lost coin and the lost son, are rebukes to the scribes and Pharisees.

The lost refers to people of the nation of Israel.

Jesus primarily came to Israel to call them to repent:

Mat 15:24 But He answered and said, "I was not sent except to the lost sheep of the house of Israel."

Mat 10:5 These twelve Jesus sent out and commanded them, saying: "Do not go into the way of the Gentiles, and do not enter a city of the Samaritans.

Mat 10:6 But go rather to the lost sheep of the house of Israel.