If both the male and female ends of a fitting are the same size, what is this type of fitting called?

When both the male and female ends of a fitting are the same size, it is commonly referred to as a standard fitting. Standard fittings are designed for connections that require the same diameter on both ends, allowing for a direct and secure attachment of pipes or other fittings without any size adjustment. This uniformity is essential for ensuring that fluids can flow freely without restrictions that could occur if there was a size mismatch, as might be the case with reducing fittings or adapter fittings.

Reducing fittings, on the other hand, are specifically designed to connect pipes of different diameters. Adapter fittings serve to connect dissimilar types of pipes or fittings, often involving different materials or connection types, such as transitioning from PVC to metal. Regular fitting is a general term and does not specifically identify a fitting with equal sizes. Thus, "standard fitting" most accurately describes the situation presented in the question.