I believe that the words "learning" and "retention" are synonyms. For example, if I learn how to redirect a folder to a new server in April, then forget how to do it again in September, I can't really say I learned it, can I? I learned how to speak English, I learned my multiplication tables, I learned how to read. I can use the word learn because I retained that knowledge. These units of knowledge are permanently imprinted in my long-term memory.
So what do we call the phenomenon of being taught something that we understand and apply but fail to retain in the long-term? I usually call it "exposure." Much of what is taught in school isn't really learned. Students are exposed to certain skills and knowledge, but 5 or 10 years later can not remember it.
When it comes to learning music or a foreign language, even a little exposure supposedly enables people to learn more deeply later in life. This may be true in other disciplines as well. Students who are taught logic may forget they even heard the word syllogism, but they may have an unconscious understanding of how to construct a logical argument.
This brings up the question: is the goal of school exposure to a variety of skills and knowledge or is the goal deeper and lasting learning? Is our job as teachers about opening neural pathways to enable learning later in life or is it about having students graduate with a conscious understanding of specific systems, skills, and information?