trajectory and distance are possibilities (however this is a qualified answer, I believe helpful in terms of a English word a users would likely understand was addressing what you ask for but not necessarily precise in any science orientated way)
I think you should use two words for clarity like Opponent Trajectory and Opponent Distance (as distance really is key in a users mind .. or if the player has multiple ships it would be enemy 1 distance from ship one ... (ok -getting away from english there)
Edit: internally, without going into computing, you are almost certainly going to want to track any entities speed in each direction like (dx, dy, and dz(if three dimensional)) as well as the entities absolute position x, y, and z - although, there are choices there too. You might have an array of enemy options each with their hahsed or object x, y, and z locations and x, y, and z speeds)
"Opponent Trajectory" or "Opponent Distance" (or Enemy ) might word better for the user however they would likely be computed values with multiple values needed to be presented or used to give the player a description they could understand to the degree of precision you choose to give.
How you would tell an player in a human readable way what the opponent trajectory is it problematic. Certainly you could subtract your X, Y, and Z speed from each of those for them and present relative speed in each direction but I don't think many players could grok both and x, y, and z position and relative difference in speed in a meaningful way.
With enough computations and array of best fits you might come up with ways of letting them know if the ship is getting closer or outwards to your trajectory and which direction on the horizontal plain from their trajectory the other ship is traveling by a clock point (i.e. 2 o'clock would be from the ne if you were flying at a constant altitude but also 2 o'clock in any 3d environment flying in a straight line .. but that would also require an 'above' or 'below" ... and it would be rough to use simple words to know if they were closing on your path or closing on your present location etc)
Still, I think "trajectory" is a good human description of that relative direction of travel even if it would be far from a single value unless you had a list of approximations ( "Foward and Outbound at 2'Oclock", "Closing From Behind and Below to Intercept") .. I am sure there are some flaws in those but .. again the word might work even though it wouldn't likely be a data store itself.