存在（英语：Existence），又译为实在、存有，是一个哲学概念，按照20世纪西方存在主义哲学家的定义，是具有难以改变，但能够改变的特性。而相对于存在的不存在（虚无）则是具有难以控制，但能够控制的特性。而两者的界别存在于（无限线）的正反面上。但此界别中的正反面本不存在，故分辨的方向只在乎于其外延对该物的影响。

贝克莱认为，无法被感知的事物（包括被神感知），就无法证明其存在，故“存在就是被感知”。

黑格尔认为，存在不过是绝对理念的一个最简单的规定；存在主义所说的“存在”，实质上仅仅是个人的主观存在。

Existence is commonly held to be that which objectively persists independent of one's presence.

Ontology is the philosophical study of the nature of being, existence or reality in general, as well as of the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology deals with questions concerning what entities exist or can be said to exist, (for instance: "Does the stellar structure UDFj-39546284 exist?"), and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences. A lively debate continues about the existence of God.