what's a sharp filter

how does it looks like? Is it a bandpass filter like SAW or is it a notch or LP or HP? This one book I'm reading keep mentioning but I just couldn't imagine how it looks like.

please read the bool DESCRETE signal processing from openhime

I don't have that book. I don't need deep understanding of that filter either. I just just need a quick overview of how does its characteristic looks like because I googled and I found none descriptive definition.