An airdrop (in most cases) is a marketing strategy that distributes a cryptocurrency or token (often for free) to user wallets to promote its’ use. This promotion uses network affects to gain attention. In the United States, airdrops have raised questions about tax liabilities and whether it can be considered income.
Examples: Airdrops can also be the result of hard forks which automatically make parent coin holders eligible for new coins/tokens. Examples include Bitcoin Cash, Bitcoin SV, Bitcoin Gold, and Litecoin Cash.