Lines are broken as result of one of two conditions. The first
condition is the presence of an explicit line breaking
character. The second condition results from a formatting algorithm
having selected among available line break opportunities; ideally the
chosen line break results in the optimal layout of the text.
NO-BREAK SPACE is the preferred character to
use where two words should be visually separated but kept on the same
line, as in the example of a currency value.
Entity defined for a non breaking space:
<!ENTITY nbsp " ">
Reference: