A wave condition in which a swell rises very quickly as it passes from deeper water to shallow water. A radical shoaling process caused by an extreme variation in water depth as the swell hits the shallow reef or ocean floor. Often creates very hollow and intense waves that appear to grow suddenly in height; thus "jacking up".

Jack Magnet
Shallow underwater obstacle that focuses swell energy in a very specific, compact area causing a wave to suddenly jack and throw. The result can be an unpredictable wave. Usually caused by a large sandbar or rock/reef but can also be man made.

Jag
The rest period after a long or intense surf session.

Jamblo
Riding a wave at maximum speed on a left hand break, usually at sunset - last wave of the day.

Jammin'
Having a good time. Listening to your favorite tunes.

Jazz
Jazz the Glass: Means to ride the waves.

Jersey
The molded inner material that holds the neoprene together, made from any combo of polypropylene, nylon, titanium, etc.

Jetty
Structure bordering both sides of an inlet, river mouth, etc. Consists of rock, caissons or concrete. Enables waves to lineup and break consistently as it travels down structure.

Jivel 
When everything goes wrong.