Bomb Defuse is an original mode in Counter-Strike.
Bomb Defuse are modes that have maps where the main objective of the Counter-Terrorists is to prevent the Terrorist from planting the C4. If this objective failed, the counter-terrorist will have to defuse the C4 before it explodes to win the round. The terrorists' main objective is to either kill the counter-terrorists or a success bomb at a bombsite.
The C4 can only be planted at a bombsite. Which there are usually two on a map and assigned as Bombsite A and Bombsite B respectively. In this mode, counter-terrorist will have to buy a Defuse Kit to decrease the time taken for bomb defusal.