Cutting means eating fewer calories than you burn for a period of time to lose body fat while keeping as much muscle as possible. In bodybuilding a cut often follows a bulk.
What sets it apart from an ordinary diet is the goal. It is not just about weighing less, but about carrying less fat with the same muscle. That is why rate, protein and training matter as much as the deficit itself.
The right rate
Helms, Aragon and Fitschen (2014) recommend that natural bodybuilders eat so that body weight drops by about 0.5 to 1 percent per week. In a study of elite athletes by Garthe et al. 2011, lean mass increased only in the group losing 0.7 percent per week, not in the group losing 1.4 percent. The leaner you already are, the better the slower end of the range fits.
Example: At 80 kg trend weight and 0.7 percent per week, you lose a little over 0.5 kg a week. Losing 5 kg then takes roughly nine to ten weeks. This is a worked example, not a prescription.
Keeping muscle
Heavy training gives your body a reason to hold on to muscle. Keep your loads and exercises as close to the same as you can, and trim volume a little if recovery suffers. Protein can sit toward the upper end during a cut.
The first week is misleading
With fewer carbs, your glycogen stores empty along with their water, so the scale drops faster at first than your deficit explains. Judge your rate by your trend weight, and not before two to three weeks.