When I need to talk about an orbital element that is not osculating and that it is the true/actual element at a given instant, I use the term "actual": actual perigee, actual eccentricity, actual semi-major axis, ... I mean that that perigee is not the osculating perigee, but it is the smallest radius vector reached by a given satellite at a given time.

Is there any official terminology for what I call "actual"?

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    $\begingroup$ And how are you going to determine that, along with eccentricity, semi-major axis, ...? You need to realize that orbital elements are a fiction, and fiction and "actual" do not go hand in hand. $\endgroup$ Dec 29 '19 at 17:28
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    $\begingroup$ The only way you can truly find the actual periapsis is to observe it at the time it happens. At that time, you can predict (i.e., make an educated guess regarding) when / where apoapsis will occur. At any point in time, you can predict (i.e., make an educated guess regarding) when / where the next periapsis and apoapsis will occur. But they are ultimately educated guesses -- i.e., not "actual". $\endgroup$ Dec 29 '19 at 18:44
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    $\begingroup$ @Cristiano The problem: suppose the position of the perigee changes for the next orbit? Do you just increment it suddenly when that happens? Perhaps you could say "orbital elements for a specific orbit" or something, meaning you've observed them for one specific orbit. $\endgroup$
    – user7073
    Dec 30 '19 at 16:15
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    $\begingroup$ @Cristiano - It was a brain fart. I know full well what argument of perigee is. Dealing with low Earth orbit and all of the weird idiosyncrasies that result has been part of my job for 30+ years. $\endgroup$ Dec 30 '19 at 19:49
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    $\begingroup$ And I would never consider calling anything that results from using TLE+SGP4 "actual". $\endgroup$ Dec 30 '19 at 19:50

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