This tutorial by shows us how to add a swap on FreeBSD 9.0 and 10.0 or later.

Fig.01: How to create a swap file on FreeBSD version 9.X and Earlier Commands

I need additional swap space to improve my FreeBSD Unix server/desktop performance. How do I add a swap file to FreeBSD system using command line options without creating a new partitions? How do I encrypt swap space on a FreeBSD Unix server for security purpose?

Tutorial details
Difficulty Intermediate (rss)
Root privileges Yes
Requirements FreeBSD
Estimated completion time 10m

A swap is nothing but space or file on a disk that can used as virtual memory. In FreeBSD and Unix-like operating systems, it is common to use a whole partition of a hard disk for swapping. When a FreeBSD based server runs out of memory, the kernel can move sleeping or inactive processes into swap area. A dedicated Swap partition goes a long way to avoid system freeze but if you notice you are running out of RAM or your applications are consuming too much of it then you may want to setup a swapfile. This guide helps you add a swap space on FreeBSD based Unix server.

Check out the full tutorial: http://www.cyberciti.biz/faq/create-a-freebsd-swap-file/