(angular measure of how much an optical instrument can view)
The field of view (field of vision, or just field, or
FOV) of an astronomical observation is the region of the
celestial sphere being observed at an instant.
The term is also used for telescopes (its field of view),
meaning the shape of such observations through it (sized to match
the FOV of an actual observation).
The actual FOV generally depends upon the instrument in use and
that of a particular instrument is often a square within the
telescope's circular FOV and may be much smaller; for example,
JWST's instruments' FOVs are each on a different small part
of the theoretical FOV provided by JWST's mirrors.
The term field of view is also used for a quantification of such
regions (often what is meant by a particular telescope's FOV)
consisting of the region's angular area, aka solid angle,
often given in square degrees or square arcminutes, but sometimes
in degrees or arcminutes, which can mean a side, a diameter,
or some such dimension giving a notion of the size.
Example telescope FOVs:
HSTACS using WFC (wide field channel): 11 square arcminutes.
same, utilizing its HRC (High Resolution Channel): 750 square arcseconds.
JWST: varies by instrument over a range of 5 to 10 square arcminutes.
The term field of view is also occasionally used to refer to the
range of surveys, similarly indicating the portion
of the sky it covers.
The term wide field is commonly used to indicate a relatively large FOV
("relatively large" given the particular instrument type and/or purpose).