The document discusses software aging, which refers to the progressive degradation of software performance over time. It notes that software aging can occur even if the software itself is not changed, simply because needs and expectations change. The document outlines two types of aging: lack of movement, where the software is not updated to meet changing needs, and ignorant surgery, where changes are made by people who do not fully understand the software. It suggests several factors that can cause aging, such as environmental changes, competition, and increasing complexity, and recommends strategies for preventing aging, such as documentation, planning for future changes, and periodic updates.