In singing, a phrase designates a musical entity that is sung on one breath; thus, it ends when you breathe again. In analogy, in instrumental music, a phrase is a meaningful musical entity with start, climax, and end; it represents „the tonal motion towards a goal“.1) This entity must be discernible during performance. This can be achieved mainly by varying dynamics (volume and accentuation). To find out the correct phrasing for a piece of music, it is helpful to try singing the piece.
In this context it should be mentioned that it is not at all correct to say that within a piece of music, there are sometimes phrases. In fact, absolutely every note of a piece is part of a phrase.2)