以及 (yǐ jí) is a formal conjunction meaning 'and' or 'as well as', used to connect nouns, noun phrases, or clauses—especially in written Chinese and formal speech. It carries a slightly more literary or precise tone than the common 和 (hé), and often implies an additive, non-exhaustive list where the second item may be of equal or slightly lesser importance. The character 以 means 'by means of' or 'with', and 及 means 'to reach' or 'as well as'; together, they convey the sense of 'extending to include'. Unlike 和, 以及 is rarely used to join verbs or adjectives.

This word appears frequently in academic writing, official documents, news reports, and formal presentations. It helps create balanced, polished sentence structures—especially when listing multiple items where clarity and hierarchy matter. Learners should avoid using 以及 in casual conversation; it sounds stiff or overly serious among friends. Also, never place it at the beginning of a sentence—it always connects elements within a clause.