similar

Similar is primarily an adjective describing resemblance: one thing is similar to another if it shares features or appearances without being identical. The term is used in ordinary language to indicate partial likeness and can apply to objects, actions, ideas, or patterns. In more formal contexts, similarity implies a calculable relation between entities rather than a vague likeness.

Etymology: similar derives from the Latin similis meaning “like,” and it entered English in the medieval period via Old French and Middle English. The related noun similarity and the adverbial form similarly come from the same root.

In geometry, similarity is a formal relation between figures. Two plane figures or solids are similar if there exists a similarity transformation that maps one to the other. Such a transformation consists of translation, rotation, reflection, and uniform scaling, preserving angles while changing size. Corresponding lengths differ by a constant scale factor; congruent figures are similar with scale factor 1. For triangles, classic criteria AA (two angles equal), SAS (two sides in proportion with the included angle equal), and SSS (all sides in proportion) establish similarity.

In other fields, similarity underpins measures of likeness. In data analysis and computer science, similarity (or similarity measures) quantifies how alike two items are, using methods such as cosine similarity, Jaccard similarity, or distance-based kernels, and it supports clustering, retrieval, and recommendation tasks.

Related concepts include resemblance and likeness, while in mathematics the notion of a similarity transformation is central; in language, simile is a related device that expresses likeness using like or as.