Explain what RES means in the process display in the htop utility on Linux?

In the htop utility in Linux, the RES column shows the size of resident (physical) memory that a process occupies in RAM. This is the amount of memory that is actually loaded into RAM and not swapped out. Unlike virtual memory (VIRT), RES reflects the part of memory actively used by the process and located in physical memory, which is important for assessing the actual load on the system.