You may have seen some
Webster_1913 dictionary writeups that mention "Mohammedanism" (like the one below). Usually it's found in old
Comparative religion books, usually European, as that was where the name was
coined. The West kept that name for centuries, relatively recently the name has faded.
What they really mean is Islam, the true name of the religion, as called by its followers. You should never use the term Mohammadanism to refer to Islam. It's inaccurate as well as offensive to Muslims. It's inaccurate because Muslims say that Muhammad(pbuh) did not found the religion, God did. Muhammad(pbuh) was only a messenger. It's also offensive because it implies that Muslims worship Muhammad(pbuh), when he was only a Prophet and a Messenger for the religion.
Naming Christianity after Christ is ok, since Christians believe that Christ is God. But calling Islam "Mohammedanism" is like calling Christianity St. Paulism. The Western scholars gave it that name because they were following a pattern:
Christ Worshipper = Christian
Buddha philosophy follower = Buddhist
But Islam does not teach the worship of Muhammad. He was only a man who was given a mission to teach people about God. Muhammad was not God. Muslims respect Muhammad, and add "peace be upon him" when speaking his name, and give the worship to God.
If there is any doubt over who is more important, Muslims are aware that Muhammad died like every other human has or will, while God is ever-living and deserves the worship.