The Psycho Tech 5/4+ represents the best of the best in winter wetsuits from O'neill. Featuring flexbility neopreme, welded seams and a thermal lining for a high performance wetsuit with a reputation for durability. 
 
Here's what O'neill have to say:

O'Neill's Psycho Tech combines our exclusive TechnoButter 3 and added heat-generating TechnoButter Air-Firewall. It's offered in a wide range of styles with the hi-tech features you need to stay warm and toasty in any condition.

Key Features
-F.U.Z.E. Closure (Chest Zip)
-Redzone Water Resistant Zipper
-Anti-Flush Barrier (Z.E.N. Zip)
-360° Barrier With Drainholes (F.U.Z.E.)
-Super Seal Cuffs

Construction
-Stitch-less Seam Technology
-Fluid Seam Weld
-Arms/ Shoulders: TechnoButter 3
-Chest / Back: TechnoButter 3 Air-Firewall Smoothskin
-Thigh/ Butt/ Shins/ Calf: TechnoButter 3 Firewall
-PLUS (+) = 0.5mm Extra thickness for even more warmth
-Aqua Alpha Solvent Free Neoprene Lamination
-Eco Carbon Black in Neoprene