The concept appears to be a fairly recent invention, only a few hundred years old. It is inherently relational. It only exists in contrast to femininity. This idea that men and women are qualitatively different did not exist until the 19th century and bourgeois ideology of separate spheres. The problem with this approach is that the choice of the essential feature is arbitrary.
Positivist definitions define masculinity as that which men actually are.
In fact the terms masculine and feminine go beyond simply differences between men and women, but describe differences within each sex on issues of gender. The problem with this is that we cannot define masculinity according to a standard that only a minute, if any, number of men actually meet. Semiotic approaches define masculinity through a system of symbolic difference between masculinity and femininity. Masculinity is defined as that which is not feminine. This definition uses masculinity as the master signifier, the place of symbolic authority, femininity is defined by lack.