I'm familiar with the concept of a filter F on a partially-ordered set: a non-empty downward directed (any two elements in F have a common lower bound) upper set (all elements above an element in F are also in F) and that a filter is proper if it's not the entire partially-ordered set.
What I cannot seem to find is a consistent definition of a normal filter on a partially-ordered set. I've seen only a couple, which didn't agree.
Where can I find a solid, accepted definition of a normal filter?