pla•teau. 1. a land area having a relatively level surface considerably raised above adjoining land on at least one side. 2. a period or state of little or no growth or decline, esp. one in which increase or progress ceases: to reach a plateau in one's career.3. to reach a state or level of little or no growth or decline; stabilize. platel flat object, diminutive of plat plate ].(plă-tō′).lateau. noun. 1. upland, table, highland, mesa, tableland a high, flat plateau of cultivated land. 2. levelling off, level, stage, stability The economy is stuck on a plateau of slow growth.