What does monit consider to be memory usage?

Once can have monit monitor memory usage…

check system foo
  if memory usage > 95% then alert

Does it use free RAM, or free -/+ buffers/cache? (or something else?)

# free -m
             total       used       free     shared    buffers     cached
Mem:           998        851        146          0        114         70
-/+ buffers/cache:        666        332
Swap:         2047         54       1993

Answer

This could be get from the source of monit https://github.com/arnaudsj/monit/tree/master/process.

For linux the value is computed in sysdep_LINUX.c and comes from /proc/meminfo :

si->total_mem_kbyte = systeminfo.mem_kbyte_max - mem_free - buffers - cached;

In other words monit use as memory usage MemTotal - MemFree - Buffers - Cached.

In your case 998 - 146 - 114 - 70 = 668