- 1a : the action of extending : state of being extended
- 2a : the total range over which something extends : compass
- 3a : the stretching of a fractured or dislocated limb so as to restore it to its natural position
- b : an unbending movement around a joint in a limb (as the knee or elbow) that increases the angle between the bones of the limb at the joint
- 4: a property whereby something occupies space
- 5: an increase in length of time; specifically : an increase in time allowed under agreement or concession
- 6: a program that geographically extends the educational resources of an institution by special arrangements (as correspondence courses) to persons otherwise unable to take advantage of such resources
- 7a : a part constituting an addition
- b : a section or line segment forming an additional length
- c : an extra telephone connected to the principal line
- d : a [length] of natural or synthetic hair that is worn attached to one's natural hair
- e : a series of usually three or four characters following a dot at the end of the name of a computer file that specifies the file's format or purpose
- 8: a mathematical set (as a field or group) that includes a given and similar set as a subset
In metaphysics, extension is, roughly speaking, the property of "taking up space". René Descartes defines extension as the property of existing in more than one dimension. For Descartes, the primary characteristic of matter is extension, just as the primary characteristic of mind is consciousness. This can be contrasted with current conceptions in quantum physics, where the Planck length, an almost unimaginably tiny quantity, represents reaching that distance scale where, it has been theorized, all measurement seemingly breaks down to that which can be subsumed at this scale, as distance only, or extension.
John Locke, in An Essay Concerning Human Understanding, defined extension as "only the Space that lies between the Extremities of those solid coherent Parts" of a body. It is the space possessed by a body. Locke refers to the extension in conjunction with solidity and impenetrability, the other primary characteristics of matter.
Extension also plays an important part in the philosophy of Baruch Spinoza, who says that substance (that which has extension) can be limited only by substance of the same sort, i.e. matter cannot be limited by ideas and vice versa. From this principle, he determines that substance is infinite. This infinite substance is what Spinoza calls God, or better yet nature, and it possesses both unlimited extension and unlimited consciousness.
The property of extension has not played a significant role in philosophy roughly since the time of Immanuel Kant. Kant maintained a distinction between the mind and the body, differentiating space as the realm of the body and time the realm of the mind. He makes only cursory mention of "extension," however, and no philosophers have dealt extensively with the topic since Kant's writing.