I have always though of an Afghan as a blanket crocheted using the Afghan stitch (which is now called Tunisian crochet). My mom used a zig-zaging double crochet TBL pattern to make some thick blankets that she called Afghans and which we used like quilts or comforters (the top bed covering that is warmer and more ornate in pattern, design, color or construction).
Lap-ghans are a blanket made smaller for covering your lap and or legs while you sit in a chair or on a couch. But I think construction method (knit or crochet stitch) was non-specific.
Like Jan said, common usage of "Afghan" in publications has slipped away from meaning the specific construction that first determined the name Afghan (especially since the stitch once called by that name is now call Tunisian).
The Afghan (Tunisian simple stitch) was thicker than common crochet stitches and made for a warmer blanket.
Quilts are named for their construction technique; they are quilted layers of fabric and batting.
To my mind, in in my opinion, blankets are thinner than Afghans and do not indicate their method of construction unless an adjective form is used such as in "a Granny Square blanket."
Baby blankets are made with baby weight yarn, so would a blanket made with sport weight be a Sport Blanket?
How about one made with Aran weight yarn?
I can here my younger children saying, "I don't want a chunky
blanket! I want a warm, soft one like you made for ----!"