In the world of cryptocurrencies word "Airdrop" imply the distribution of tokens for performing some simple task. For example, when user participates in ICO the creators of ICO can declare Airdrop for performing a simple task like to make a repost, write a unique comment, subscribe to an account, and so on. User will be credited with free tokens after the assignment. So airdrop is a distribution of tokens for performing simple tasks.
