Wiktionary
non-breaking space
n. (context computing English) A variant of the space character that prevents an automatic line break (line wrap) at its position.
Wikipedia
Non-breaking space
In word processing and digital typesetting, a non-breaking space (" ") (also called no-break space, non-breakable space (NBSP), hard space, or fixed space) is a space character that prevents an automatic line break at its position. In some formats, including HTML, it also prevents consecutive whitespace characters from collapsing into a single space.
In HTML, the common non-breaking space, which is the same width as the ordinary space character, is encoded as or  . In Unicode, it is encoded as U+00A0.
Non-breaking space characters with other widths also exist.