The D'Alembert is a betting system which is very like the Martingale system. The D'Alembert system tells you to raise your bet by one unit after a loss and lower your bet by one unit after a win. The sequence is:
For example, your first bet is $5 (one unit). You have lost so your next bet must be $10. If you win you must bet $5 again. If you have lost your second hand you must bet $15. If your second hand wins, your bet will be $10. The difference with The Martingale is that you lower only by a unit, and do not revert to the original bet.
The D'Alembert is a betting system makes sense in the long run only.
There are some variants of the D’Alembert. The Contra D'Alembert System wants you to add one unit after each win and lower after each loss.