Although there is no firm definition of this term, the expression dead money is used to describe any chips in the pot that are seemingly up for grabs.
One example of dead money could be when the player in the Big Blind is not at the table but has already posted the blinds. Since he is not there to defend his BB and will be automatically folded, it is considered dead money. The same can be said about players folding out of turn and in other instances when the chips are already in the pot, but noone is not going to defend that.
Another application where the term dead money can come into play is the entire table limps to you, and you are the last to act. You will be strongly inclined to raise with any reasonable hand because there is so much dead money in the pot, i.e., players have put in chips in the middle with hands that are likely quite weak and will give up facing aggression.