From AMS Glossary
A neutral surface is that surface along which fluid particles can be exchanged without doing any work against gravity.
For many oceanographic purposes it is sufficiently accurate to assume that this surface is defined by a constant value of potential density. In ocean mixing studies, however, this definition will often not be sufficiently accurate due to lateral variations of temperature, salinity, and pressure along a potential density surface. These variations will cause fluid particles from different points within the potential density surface to migrate slightly off that surface when they are displaced laterally.