As far as I can tell, the CSS 2.1 definitions of line box and inline box/inline-level box have not been repeated as formal definitions in any of the CSS level 3 text layout specs.

I'm not sure whether the intention is to place all the definitive definition for them in Basic Box Model 3, but for "line box" in particular, the current rough & outdated draft points to CSS Text 3, which no longer defines the term. CSS Inline 3 would seem to be a logical place for that definition -- it is where you get redirected if you follow http://www.w3.org/TR/css3-linebox from an ancient draft spec -- but it never defines its terms.