Self-absorption is the noun for being self-absorbed: the state, habit, or stretch of time when a person is wrapped up in themselves and the things that affect them. The describing word is self-absorbed; this word names the thing. It is a fairly formal noun. In easy talk people often skip it and just say “he is so self-absorbed,” rather than “his self-absorption.” You will meet it in essays, in careful talk about character, and sometimes in psychology-adjacent writing. It is not a medical term in everyday English, even when it sounds like one, and it is not “absorption” in the science sense of a sponge taking in water — though the picture underneath is close: the person has been taken in by their own life. Use it when you mean the condition, not when you only need an adjective.