Merchant Levels are designated by the PCI Standard as follows:
Level 1: Any merchant, regardless of acceptance channel, processing over 6 million Visa transactions per year; that has suffered a hack or an attack that resulted in an account data compromise; that Visa, at its sole discretion, determines should meet the Level 1 merchant requirements to minimize risk to the Visa system.
Level 2: Any merchant, regardless of acceptance channel, processing 1 million to 6 million Visa transactions per year.
Level 3: Any merchant processing 20,000 to 1 million Visa e-commerce transactions per year.
Level 4: Any merchant processing fewer than 20,000 Visa e-commerce transactions per year, and all other merchants, regardless of acceptance channel, processing up to 6 million Visa transactions per year.
Source: www.pcistandard.com