What does it mean that Satan is prince of the power of the air (Ephesians 2:2)?
Answer: "And you were dead in the trespasses and sins in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience" (Ephesians 2:1–2, emphasis added). In this text the apostle Paul describes Satan first as a "prince" with power, because he has authentic power in the world (1 John 5:19). This power has been given him by God (Luke 4:6). Satan has power over some illnesses (Luke 13:16; see also 2 Corinthians 12:7—it's unknown if Paul's "thorn" was an illness or something else). In some sense, Satan has power over death (Hebrew 2:14). The reason Satan is called a prince rather than a king is because there is only one King—Jesus Christ (1 Timothy 6:15).
Satan also has power over some people. The "sons of disobedience" referred to in Ephesians 2:2 are those who have not trusted Christ as Lord and Savior (cf. Acts 26:18; 2 Corinthians 4:4; Revelation 13:12). The demons are also under the rule of Satan (Matthew 12:24), and one of his titles is "prince of demons" (Matthew 9:34). Satan has a kingdom (Matthew 12:26) and a throne (Revelation 2:13). Satan is called a prince because he is a ruler and possesses power to manifest evil in the world through influencing people and commanding demons.
"The air" in Ephesians 2:2 may refer to the invisible realm above the earth where Satan and his demons move and exist. This space, of course, is the location of the earth's atmosphere or "air." In Ephesians 6:12, Paul writes, "We do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places." This evil realm called the "air" could be an actual locality, but it could also be synonymous with the "world" of John 12:31. This whole world is Satan's domain (Matthew 4:8–9).
Although Satan has power and authority in the current world system in which we exist, his power is limited, always under the sovereign control of God (Job 1:12), and it is temporary (Romans 16:20). God has not revealed all of the why's and when's concerning Satan's rule, but He has made it clear that there is only one way to escape the power of Satan's dominion, and that is through His Son, Jesus (Acts 26:18; Colossians 1:13–14). It is Jesus who, speaking of the impending cross, declared victory: "Now the prince of this world will be driven out" (John 12:31).