An antenna is said to be resonant if its input impedance (or feed point impedance), Z=R+jX, is purely resistive, which means its reactance X is exactly zero.
The input impedance of a half-wave dipole is approximated by the equation:
Z=73.13+j*42.55+j*We*k*delta,
where We and k are some constants, and delta is the excess length of an antenna over half the wavelength. Since both We and k are positive, we can make X to be zero by shortening the element slightly, usually a few percent depending on the diameter of the element.
The question here is what is the physical and practical meaning of the antenna being in resonant.
References:
[1] “Antenna and Radio propagation (in Japanese)” by Yasuto Mushiake,
[2] MMANA-GAL basic.