The functional group in the compound CH3-CH2-O-CH3 is an ether.

Here's why:

* Ethers are characterized by an oxygen atom connected to two alkyl or aryl groups.

* In this compound, the oxygen atom is bonded to a CH3 (methyl) group on one side and a CH2CH3 (ethyl) group on the other.

Let me know if you have any other questions about functional groups!