What does bi symmetry mean?

Answer 1

Bilateral symmetry means something has symmetry across one plane (known as the sagittal plane, and directly down the centre of their body), which means one side of their body approximately mirrors the other side.

It is just like the line of symmetry in maths for 2D shapes such as an oblong or isosceles triangle.

Examples of organisms that have bilateral symmetry are all mammals, birds, fish, reptiles and amphibians, as well as insects, e.g. ants and butterflies; arachnids, e.g. Spiders and crustaceans, e.g. crabs and lobsters.