Slowed is a condition.

• The creature's speed becomes 2 if it was higher than that. This speed applies to all your movement modes, but it does not apply to forced movement, teleportation or any movement that doesn't use the creature's speed. A creature that is slowed while moving must stop moving if it has already moved 2 or more squares.
• The creature can't benefit from bonuses to speed, but may use powers or actions that allow you to move farther than your speed (e.g. the run action)

The FAQ contains two pieces of clarification for the slowed condition ^{[previous edition support]}:

• If a character is wearing heavy armor, his speed doesn't get reduced from 2 to 1, as a slowed character has a speed of 2 regardless of other penalties to speed.  This is found in the Player's Handbook section of the FAQ.
• When slowed and using Plague of Rooted Majesty, your speed is reduced to 0, because Plague of Rooted Majesty reduces your speed by 2. This is found in the Forgotten Realms Player's Guide section of the FAQ.