Crossover frequency and resonant frequency
You are correct about resonant frequency.  The "crossover" frequency 
depends on the topic.  

As @John D pointed out in the comments, for controls, it's where the amplitude crosses 0dB.
In audio, it is more related to where gain lines intersect, such as where mid-range and tweeters "crossover" and their gains are equal.  See the image, where the blue and green cross, this is a crossover point in audio.  Note this is irrespective of the magnitude of the gain at this point, in fact it could be positive, or very negative.


The -3dB point, is where the output signal amplitude is half of the input amplitude.