Zygomorphic

Zygomorphic or bilateral (or irregular) - also known as monosymmetric.

divided into equal halves along one (longitudinal) plane only, or in other words,
divisible through the centre of the flower in only one longitudinal plane for the halves of the flower to be mirror images.

Illustration from Clarke, I. & Lee, H. (2003) Name that Flower, Melbourne University Press, Victoria.