user31734: My question is not exactly about what advice you have for me so that I can learn openings better, although that is indeed a goal of mine.
My question is more about wanting to have either an abstract description of the process of learning openings (with various degrees of complexity) or empirical studies of ways in which players actually study openings.
If you know of any sources, especially academic sources, please include citations and/or links.

user13012: I would just add to the excellent answer of @David that what it means to learn an opening depends to some extent on the opening.
The London system used to known in the UK as the Businessman's opening because a busy business man need not devote much time to it. Just put your pieces on sensible squares where they cooperate and then start playing. This is of course not quite as true as it used to be, neither is it as true as it used to be of the Giuoco Piano, but there are still plenty of lines where common sense will see you through at all levels except for the very highest.
On the other hand, both players may elect to steer into a line that is the topic of current GM debate. Then you must stay abreast and even get ahead if you think you can.

user18809: What "learning an opening" means varies a lot depending on your skill level.
A total beginner may think they've "learned" the Spanish if they know how to get to 1.e4 e5 2.Nf3 Nc6 3.Bb5
A weak-ish amateur will learn a couple of lines against the main responses and be happy with that.
An average club player probably knows the typical middlegame plans that arise from the different liens.
An enthusiastic amateur has probably read several books and memorized many lines or even full games from them. They're fully familiar with the plans/ideas of the middlegame and will know some of them in great detail with lots of examples in mind.
Finally, a professional player will probably not consider they "know" an opening if they haven't prepared some novelty of their own.
In conclusion, learning is a gradual process, so it doesn't make sense to categorize into "I know this opening" and "I don't know this opening". Play, analyze, study and increase the scope of your knowledge bit by bit. There is no a "formal definition" for what learning an opening is because there can't be any useful universal definitions for that.

31734: <p>My question is not exactly about what advice you have for me so that I can learn openings better, although that is indeed a goal of mine.</p>
<p>My question is more about wanting to have either an abstract description of the process of learning openings (with various degrees of complexity) or empirical studies of ways in which players actually study openings.</p>
<p>If you know of any sources, especially academic sources, please include citations and/or links.</p>
 13012: <p>I would just add to the excellent <a href="https://chess.stackexchange.com/a/40745">answer</a> of @David that what it means to learn an opening depends to some extent on the opening.</p>
<p>The London system used to known in the UK as the Businessman's opening because a busy business man need not devote much time to it. Just put your pieces on sensible squares where they cooperate and then start playing. This is of course not quite as true as it used to be, neither is it as true as it used to be of the Giuoco Piano, but there are still plenty of lines where common sense will see you through at all levels except for the very highest.</p>
<p>On the other hand, both players may elect to steer into a line that is the topic of current GM debate. Then you must stay abreast and even get ahead if you think you can.</p>
 18809: <p>What &quot;learning an opening&quot; means varies a lot depending on your skill level.</p>
<p>A total beginner may think they've &quot;learned&quot; the Spanish if they know how to get to <strong>1.e4 e5 2.Nf3 Nc6 3.Bb5</strong></p>
<p>A weak-ish amateur will learn a couple of lines against the main responses and be happy with that.</p>
<p>An average club player probably knows the typical middlegame plans that arise from the different liens.</p>
<p>An enthusiastic amateur has probably read several books and memorized many lines or even full games from them. They're fully familiar with the plans/ideas of the middlegame and will know some of them in great detail with lots of examples in mind.</p>
<p>Finally, a professional player will probably not consider they &quot;know&quot; an opening if they haven't prepared some novelty of their own.</p>
<p>In conclusion, learning is a gradual process, so it doesn't make sense to categorize into &quot;I know this opening&quot; and &quot;I don't know this opening&quot;. Play, analyze, study and increase the scope of your knowledge bit by bit. There is no a &quot;formal definition&quot; for what learning an opening is because there can't be any useful universal definitions for that.</p>
