In biology, cell theory is a scientific theory which describes the properties of cells. These cells are the basic unit of structure in all organisms and also the basic unit of reproduction.
The doctype declaration should be the very first thing in an HTML document, before the <html> tag. The doctype declaration is not an HTML tag; it is an instruction to the web browser about what version of the markup language the page is written in. The doctype declaration refers to a Document Type Definition (DTD).
A declaration is often used in order to access functions or variables defined in different source files, or in a library. A mismatch between the definition type and the declaration type generates a compiler error. For variables, definitions assign values to an area of memory that was reserved during the declaration phase.