An unlimited data plan is one that puts no limit on the amount of data used on a cell phone or tablet. Unlimited does not necessarily mean unlimited at full speed.
Unlimited data plans can come with restrictions. Some data plans are truly unlimited and only the speed is throttled. Usually, data is available at the maximum speed to begin with and, after a certain amount is used, the speed is slowed down. How much and when the speed is slowed depends on the carrier and the plan. There are also plans that do not throttle the speed of data, but other restrictions apply.
International data may or may not be included in unlimited plans, and these plans may only be available at specific speeds despite higher speeds being offered by the company. Some carriers only request that users adhere to their "fair use policy." Many unlimited plans also restrict tethering, or connecting the device to a computer or tablet for full Internet access, either by forbidding it or by limiting the amount of data that can be used for tethering.
Some prepaid plans may offer the option of unlimited data as well. Restrictions are much the same as with a contract plan, but can also include restrictions regarding which devices are capable of using the unlimited data plans.