Two different figures are similar when they are only different in size
The corresponding parts are proportional.
Each side of one is obtained by multiplying the same side of the other by a fixed number called the ratio of similarity
If lines a, b and c are parallel and cross two other lines r and s, then the segments inside are proportional
Plans and maps are similar to the reality they represent
Sizes and distances are important, as well as the distribution of areas
The scale is the ratio between each reproduced length and the same lenght in reality (the ratio of similarity)