A-scan = RF or rectified waveform
B-scan = profile view
C-scan = planar view (like an x-ray)
You might want to check the "Imaging Basics" section of our phased array tutorial for some examples of each. See http://www.olympus-ims.com/en/ndt-tutorials/phased-array/
I'm not certain of the historical derivation of A-scan -- the term was in use long before I started working with ultrasonics thirty years ago -- but I suspect that "amplitude scan" was informally abbreviated into A-scan, and then B-scan and C-scan followed in sequence as those plotting methods were developed. But there may be an NDT historian here who will correct me!