Worldline

Any particular worldline is a (generally nonempty) set,
each of whose members has in turn observational content
(colloquially: concerning the world of what this same worldline represents),
together with the linear order which is evaluated, between pairs of its members, from the observational content.

Therefore synonymous with, or an abstraction of, observer.

Instructions on commensurate relations between worldlines or their members, and results of their evaluations, are collectively referred to as Physics.

An alternatively weaker notion of worldline, [1] (http://216.239.33.100/search?q=cache:TUFFVOJm1hgC:library.thinkquest.org/27930/worldlines.htm+worldlines&hl=en&ie=UTF-8), has been suggested.