Abarim Publications' online Biblical Greek Dictionary
αφρος
The noun αφρος (aphros) means foam and is applied the same in Greek as in English. In the classics our noun primarily describes the foam that forms on the sea or even a river, but also the foam on the mouth of a raging beast, and on occasion froth on wine or even blood. Note that foam results from a mix of water and air, whereas water mixed with light produces a rainbow.
In the Bible our noun occurs only in Luke 9:39, where an unclean spirit takes hold of a boy and "tears (σπαρασσω, sparasso) him with foam (αφρος, aphros)".
Our noun has one derivative, namely the verb αφριζω (aphrizo), meaning to foam. This verb occurs only in Mark's version of the story of the demoniac boy (Mark 9:18 and 9:20 only).
It's generally assumed that the verb sparasso denotes a convulsing and that the foam appeared on the boy's mouth, but all the texts say is that the boy foamed, and not in which way. Still, both Luke and Mark insist on mentioning this foam, and this while a foaming mouth doesn't seem like a very crucial detail of the whole pallet of elements of a demonic seizure.
Here at Abarim Publications we surmise that the story of the young demoniac also serves as a commentary on the culture of Cyprus, whose capital (Paphos) and patron deity (Aphrodite, the Greek equivalent of Ashtoreth, in the Old Testament the primary feminine rival of YHWH) were both named after our noun αφρος (aphros), meaning foam. Similar themes occur in the book of Acts, where Paul and Barnabas are forced to rebuke the corrupt Jewish magician Elymas while trying to serve the Roman proconsul of Cyprus, Sergius Paulus (Acts 13:6-7).