What is the meaning of X in 52x CD ROM drive?

52X has nothing to do with the rotational speed of the platter. The 52X speed rating indicates the refers to the fact that the drive can read data off of the disk at 52 times the speed of an original CD-ROM drive. Original CD-ROM drives read disk data at 150 kilobytes per second, so a 52X drive reads at 52 x 150KB/s.