Similarity
Two mathematical figures are similar if they have the exact same shape, but not necessarily the same size. Their corresponding angles are identical, and their corresponding side lengths are proportional.
Example
If $\triangle ABC$ is similar to $\triangle DEF$ (written as $\triangle ABC \sim \triangle DEF$), and the sides of $\triangle DEF$ are twice as long as the sides of $\triangle ABC$, the ratio of their corresponding sides is $1:2$.