Definition of dominated
In poker, you are dominated when your opponent has one card in common with you, but his other card is stronger than yours. For example, AK dominates AQ or KJ dominates KT. When you are preflop with a dominated hand and you are all-in, your equity is usually 25%. We might also use the term if you have a pair of 8's all-in preflop that meet a pair of 9's, but usually, because of the huge equity gap, we'll say you're crushed in this situation (with less than 20% equity).
Dominated after the flop
The term dominated can also be used to refer to a postflop situation. Generally, when we talk about a dominated hand, it will be when you are all in with two hands all-in. You push all-in with AQ on A23 and your opponent calls with AK; here you are dominated. The closer you get to the river, the worse your equity will be. AK vs AQ preflop will give AQ 25% equity. On the flop, about 14% if the hand hasn't improved and on the turn, 7%. On the turn, not only are you dominated by AK, but given your equity, you are crushed!