The most general type of an expression. For example, the following are all valid types for the lambda abstraction (\ x . x):
Int -> Int Bool -> Bool (a->b) -> (a->b)
but any valid type will be an instance of the principal type: a -> a. An instance is derived by substituting the same type expression for all occurences of some type variable. The principal type of an expression can be computed from those of its subexpressions by Robinson's unification algorithm.
The principal type of these is the cylinder, with many small variations.
The Alcyonaria are so designated from their principal type, that of the Alcyons.
I have before me a coin of Stralsund, minted in the fourteenth century, with the Pheon for the principal type.
These memorials related to a history of which the dove was the principal type.
The principal type of the Euryalina is the curious and complex Asterophyton verrucosum of Lamarck.
The principal type is the Common Crocodile, C. vulgaris, which sometimes attains the length of nine or ten feet.
The principal type of soil is mud, and the mud flats comprise about 250 acres.
Ruskin considered this to be the principal type at Venice and the best in Europe of the central style of the Renaissance schools.
During most of the period the galley, with three banks of oars, was the principal type of vessel.