Resonance is when applied oscillation is in phase with natural oscillation. Then, what is resonant frequency?
Resonance is the process of driving a system to oscillate at a resonant frequency. The property of these frequencies is that they can store vibrational energy or vibrational quanta depending on the description you choose. This means that driving the system at these frequencies can create much larger amplitude oscillations than other frequencies, which disperse energy much more quickly.
A system may support many resonant frequencies, and driving the system at any of those frequencies would be cause resonance. That is, 1 system can have many resonances, each of which is associated with a resonant frequency.
Resonance is the name for the phenomenon.
The resonant frequency is the frequency at which this phenomenon occurs. The frequency of the oscillation when you observe resonance.
Frequency is just one of the characteristics of a resonance. Another one is Q factor. Others might be the mode structure of the resonant wave.