The Erya is the oldest extant Chinese dictionary or Chinese encyclopedia. Bernhard Karlgren (1931: 49) concluded that "the major part of its glosses must reasonably date from" the 3rd century BC.

Chinese scholars interpret the first title character er" (爾; "you, your; adverbial suffix") as a phonetic loan character for the homophonous er (邇; "near; close; approach"), and believe the second ya (雅; "proper; correct; refined; elegant") refers to words or language. According to W. South Coblin (1993: 94): "The interpretation of the title as something like 'approaching what is correct, proper, refined' is now widely accepted." It has been translated as "The Literary Expositor," "The Ready Rectifier" (both by James Legge), and "Progress Towards Correctness" (A. von Rosthorn). "Approaching Elegance/Refinement," Alex Kolesnikov.

The book's author is unknown. Although it is traditionally attributed to the Duke of Zhou, Confucius, or his disciples, scholarship suggests that someone compiled and edited diverse glosses from commentaries to pre-Qin texts, especially the Shijing. The Erya was considered the authoritative lexicographic guide to Chinese classic texts during the Han Dynasty, and it was officially categorised as one of the Thirteen Confucian Classics during the Song Dynasty. The best-known textual annotations include the Western Jin Dynasty Erya zhu (爾雅注; "Erya Commentary") by Guo Pu (郭璞; 276-324 CE), the Northern Song Dynasty Erya shu (爾雅疏; "Erya Subcommentary") by Xing Bing (邢昺; 931-1010), the Song Dynasty Eryayi (爾雅翼; "Wings to the Erya") by Luo Yuan (羅願; 1136-1184), and the Qing Dynasty Erya zhengyi (爾雅正義; "Correct Meanings of the Erya") by Shao Jinhan (邵晋涵; 1743-1796) and Erya yishu (爾雅義疏 "Subcommentary on Meanings of the Erya") by Hao Yixing (郝懿行; 1757-1825).

The Erya has been described as a dictionary, glossary, synonymicon, thesaurus, and encyclopaedia. Karlgren (1931: 46) explains that the book "is not a dictionary in abstracto, it is a collection of direct glosses to concrete passages in ancient texts." The received text contains 2094 entries, covering about 4300 words, and a total of 13,113 characters. It is divided into nineteen sections, the first of which is subdivided into two parts. The title of each chapter combines shi ("explain; elucidate") with a term describing the words under definition. Seven chapters (4, 8, 9, 10, 12, 18, and 19) are organized into taxonomies. For instance, chapter 4 defines terms for: paternal clan (宗族), maternal relatives (母黨), wife's relatives (妻黨), and marriage (婚姻). The text is divided between the first three heterogeneous chapters defining abstract words and the last sixteen semantically-arranged chapters defining concrete words. The last seven – concerning grasses, trees, insects and reptiles, fish, birds, wild animals, and domestic animals – describe more than 590 kinds of flora and fauna. It is a valuable document of natural history and historical biogeography.