ss vs mia
No difference in meaning: both say an enemy has left their lane. It is a question of era and habit, not vocabulary.
ss
"ss" comes from miss summoner, a phrase from the game's earliest days. It is the shortest form, so the most typed: two letters you can fit between two movements without looking away from your lane.
open the page →mia
"mia" is short for missing in action, a common english expression borrowed from military usage. Easier to read for a native speaker, longer to type, and more common among players who came from other games.
open the page →Why they get mixed up
They look distinct because they coexist in the same chat, often in the same game. They are not. What actually matters is what you add after: "ss" alone helps nobody, "ss mid 30s" tells your team where to look and for how long.
"ss" and "mia" share a single dictionary page, since they mean the same thing. ss