In Euclidean geometry, a rhombus (◊), plural rhombi or rhombuses, is a quadrilateral whose four sides all have the same length. Another name is equilateral quadrilateral, since equilateral means that all of its sides are equal. A rhombus is necessarily convex. The rhombus is often called a diamond, after the diamonds suit in playing cards, or a lozenge, though the latter sometimes refers specifically to a rhombus with a 45° angle.

Every rhombus is a parallelogram, and a rhombus with right angles is a square. (Euclid's original definition and some English dictionaries' definition of rhombus excludes squares, but modern mathematicians prefer the inclusive definition.)

The word "rhombus" comes from the Greek ῥόμβος (rhombos), meaning something that spins, which derives from the verb ρέμβω (rhembō), meaning "to turn round and round". The word was used both by Euclid and Archimedes, who used the term "solid rhombus" for two right circular cones sharing a common base.

A quadrilateral is a rhombus if and only if it is any one of the following:

A lozenge (◊), often referred to as a diamond, is a form of rhombus. The definition of lozenge is not strictly fixed, and it is sometimes used simply as a synonym (from the French losange) for rhombus. Most often, though, lozenge refers to a thin rhombus—a rhombus with acute angles of 45°. The lozenge shape is often used in parquetry and as decoration on ceramics, silverware and textiles. It also features in heraldry and playing cards.

The lozenge motif dates as far back as the Neolithic and Paleolithic period in Eastern Europe and represents a sown field and female fertility. The ancient lozenge pattern often shows up in Diamond vault architecture, in traditional dress patterns of Slavic peoples, and in traditional Ukrainian embroidery. The lozenge pattern also appears extensively in Celtic art, art from the Ottoman Empire, and ancient Phrygian art.

Seventeenth century orchards in England were planted on a symmetrical grid pattern. In 1659, philosopher Sir Thomas Browne published The Garden of Cyrus subtitled The Quincunciall Lozenge, or Network Plantations of the Ancients where he outlined mystical interconnection of art, nature and the Universe. He suggested that ancient plantations used the quincunx pattern that revealed the "mystical mathematics of the city of Heaven" and proof of the wisdom of God.

"Choose the right" is a saying or motto among members of The Church of Jesus Christ of Latter-day Saints (LDS Church) that is taught to children and used by members of the church as a reminder to act righteously. The phrase is taken from an LDS hymn which has that title.

The letters CTR, a reference to this phrase, are incorporated into a shield logo designed by Helen Alldredge, a Primary General Board member in the 1960s. In 1970, a church committee headed by Naomi W. Randall recommended that the shield be incorporated into official church material. Since then, both the phrase and symbol have been used in religious educational materials for LDS youth of Primary age. The symbol is also used in LDS culture by members of all ages, both as a reminder of the motto, as well as an indicator of religious affiliation. The CTR initials, displayed on the shield and in other forms, can be seen mainly on CTR rings as well as other types of jewelry, tee shirts, bookmarks and stationery.

According to the US Patent and Trademark Office, the LDS Church's stylized "CTR" abbreviation and shield with the CTR symbol are trademarked for use on finger ring jewelry by Intellectual Reserve. The phrase "Choose the Right," however, is not trademarked in the United States and may be used by anyone for commercial purposes.^{[citation needed]}