Used to refer to the Unix operating system (a trademark of AT&T) in writing, but avoiding the need for the ugly (TM) typography. Also used to refer to any or all varieties of Unixoid operating systems. Ironically, lawyers now say that the requirement for the TM-postfix has no legal force, but the asterisk usage is entrenched anyhow.
It has been suggested that there may be a psychological connection to practice in certain religions (especially Judaism) in which the name of the deity is never written out in full, e.g. "YHWH" or "G--d" is used.