Question

how define the convex lens and concave lens

Answer 1

$\bf{Convex\:Lens}$ are the lens which are bulged out on both sides.

In the Image - The First one is a convex lens.

Convex Lens forms real and inverted Image until the object is between its Optical Center and Focus.

It is also called as converging lens.
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$\bf{Concave\:Lens}$ are the lens which are curved inwards on both sides.

In the Image - The Second One is a concave lens.

Concave Lens always forms virtual and erect Images.

It is also called as Diverging lens.

Answer 2

Concave lens

a lens (made by joining the parts of two spherical surface ) that is thinner in middle than at the edges. It diverges the incident rays away from its principal axis

Convex lens

a lens (made by joining the parts of two spherical surfaces) that is thicker in the middle than at the edges'. it converges the incident rays towards its principal axis