available memory

The available column of free -h: the memory an application can obtain right now, including what the kernel would hand back by dropping disk cache. It is the column that answers "is this machine short of memory". free does not: a line reading 182Mi of free, 2.5Gi of buff/cache and 2.4Gi available describes a healthy machine. Linux uses memory nothing else wants as disk cache and gives it up the moment a program asks — memory left free is memory wasted. So alerting on free wakes somebody for a machine that is fine, and misses the pressure that counts: swap filling up, and a dmesg -T | grep -i "out of memory" showing the OOM killer taking a process the application logs never mention.