It's a useless term. Just drop the word, "inner" and be done. Resolution, focus, definition and detail are basically synonymous.
To be more specific, here is the Stereophile definition of inner detail: The sonic subtleties within a complex program signal, reproducible only by a system having high resolution. See "focus."
Here is the definition of detail: The subtlest, most delicate parts of the original sound, which are usually the first things lost by imperfect components. See "low-level detail." Compare "haze," "smearing," "veiling."
Here is the definition of focus: The quality of being clearly defined, with sharply outlined phantom images. Focus has also been described as the enhanced ability to hear the brief moments of silence between the musical impulses in reproduced sound.
Here is the definition of resolution: See "definition."
Here is the definition of definition: (also resolution) That quality of sound reproduction which enables the listener to distinguish between, and follow the melodic lines of, the individual voices or instruments comprising a large performing group. See "focus."
Confused? You should be.