Solidarity is the name given to a Tier 1 Legacy deck, created by David Gearhart.
Solidarity (also called simply Reset High Tide) is a storm combo deck that wins by casting a large number of blue instant spells, fueled by the mana engine of High Tide and Reset (or Turnabout), and then playing a lethal Brain Freeze on the opponent (so as to deck him or her). The deck is entirely composed of basic Islands, fetch lands, and blue instants. It was considered the best combo deck in the Legacy format roughly from its creation to 2006–7, due to its extremely high resilience and consistency.
Solidarity can usually go off by turn 4 or 5. Originally the deck would simply try to "goldfish" its opponent, its only protection against counter-magic and disruption being 4 Force of Will; the latest version of the deck employs Remand as well to slow down the opponent (or, alternatively, to play Brain Freeze twice, thus reducing the storm count needed to mill an entire deck).
Since playing Solidarity requires an excellent knowledge of timing rules, a good ability to guess the presence and amount of disruption in its opponent's hand, and the stamina to endure all the difficult choices it offers every game, it is widely believed to be one of the most difficult decks to play in the Legacy format and perhaps in the entire game.
The name "Solidarity" comes from a Limited game where David Gearhardt witnessed a player win thanks to the card Solidarity (a terrible card in itself). This would spawn a kind of a habit in Legacy to name decks based on entirely unrelated cards (see Nausea and Rabid Wombat).