The oldest and most primitive form of whatever can be called plesiomorph.
Some background info to the term:
During the 1950’s and1960’s three competing theories of classification came up:
a. Phenetic systematics (Phenetics): Organisms are classified, according to this theory, on the basis of “overall similarity”. Similarity is calculated from the presence or absence of numerous unweighted characters or character states . This method does not establish groups by inspection, but orders the lowest taxonomic units (usually species) into groups with the help of standardized procedures.
b. Cladistic systematics (Cladistics) Organisms are classified and ranked, according to this theory, exclusively on the basis of “recency of common descent”. Membership of species in taxa is recognized by the joint possession of derived (“apomorphous”) characters. Grouping and ranking are given simultaneously by the branching points.
c. Evolutionary systematics: Organisms are classified and ranked, according to this theory, on the basis of two sets of factors, 1. phylogenetic branching (“recency of common descent”, retrospectively defined), and 2. amount and nature of evolutionary change between branching points. The latter factor, in turn, depends on the evolutionary history of a respective branch, e. g., whether or not it has entered a new adaptive zone and to what extent it has experienced a major radiation.
Cladistics consists of two quite different sets of operations:
1. the reconstruction of the branching pattern of phylogeny through cladistic analysis,
2. the construction of a cladistic classification based on this branching pattern.
The most important step in the cladistic analysis is the attempt to separate characters into ancestral (plesiomorphous) and derived (apomorphous) characters.
The following terms are used to identify shared or distinct characters among groups:
• Plesiomorphy ("close form") or ancestral state, also symplesiomorphy ("shared plesiomorphy", i.e. "shared close form"), is a characteristic that is present at the base of a tree (cladogram). Since a plesiomorphy that is inherited from the common ancestor may appear anywhere in a tree, its presence provides no evidence of relationships within the tree.
• Apomorphy ("separate form") or derived state is a characteristic believed to have evolved within the tree. It can thus be used to separate one group in the tree from the rest. Within the group which shares the apomorphy it is a synapomorphy ("shared apomorphy", i.e. "shared separate form").
My conclusion: Plesiomorphic flight (if the term exists) is another term for primitive flight, but not a form of flight.