A block is a group of three Magic: The Gathering expansion sets that are centered or focused on a common storyline cycle, or arc. Typically, the first set in a block is a large expansion, and is followed by two small expansions; but, this trend has been subverted a number of times (e.g., with Lorwyn block and Shadowmoor block, which comprised two expansions each; Return to Ravnica block, which comprises two large expansions and one small expansion).  
Modern block design commenced in 1997, with Mirage block.
Ice Age and Alliances were grouped together with Homelands as a block retroactively. As Homelands is thematically and mechanically unrelated to Ice Age and Alliances, as well as being of a different storyline arc, it was later removed from the Ice Age block and replaced with Coldsnap in 2006.
List of blocks
The following list details all of the blocks in Magic: The Gathering in chronological order.
The year given in parentheses is the year in which the first set in the block was released. Typically, with a notable exception being Lorwyn and Shadowmoor blocks, Wizards of the Coast has released the first set in October of the year in parentheses and subsequent sets in the block in February and April of the following year.