The loss of fitness that follows stopping training, which happens faster for endurance than for strength and faster than most clients expect.
The timelines are reasonably well established. Aerobic fitness starts falling within about 10 to 14 days. Maximal strength holds for roughly three to four weeks and then declines slowly. Muscle size holds longer still, with visible loss after about three weeks of complete rest.
The reassuring part is that maintenance is cheap. One session per muscle per week at the usual intensity holds strength and size for months, even with volume cut by two thirds. A client going on a four-week trip does not lose the year: two full-body sessions a week with a third of their normal sets is enough.
What comes back fastest is what was lost first, and previously trained lifters regain size far faster than they built it.