What is a ladder?

a structure consisting of a series of bars or steps between two upright lengths of wood, metal, or rope, used for climbing up or down something.

A ladder is a vertical or inclined set of rungs or steps. 
There are two types: rigid ladders that can be leaned against a vertical surface such as a wall, and rope ladders that are hung from the top. 
The vertical members of a rigid ladder are called stringers or rails (US) or stiles (UK). 
Rigid ladders are usually portable, but some types are permanently fixed to buildings. 
They are commonly made of metal, wood, or fibreglass, but they have been known to be made of tough plastic.

a structure of wood, metal, or rope, commonly consisting of two sidepieces between which a series of bars or rungs are set at suitable distances, forming a means of climbing up or down.

 An often portable structure consisting of two long sides crossed by parallel rungs, used to climb up and down.

a portable framework of wood, metal, rope, etc, in the form of two long parallel members connected by several parallel rungs or steps fixed to them at right angles, for climbing up or down