צִּיּוֹן tsiyon "Zion" the word may mean "fortress", as a name it refers to Jerusalem, in biblical usage often there is particular reference to the temple mount or the old citadel, which David captured from the Jebusites. (The use, since Byzantine times, of "Zion" to refer to another hill is confusing for the modern visitor.)
"Zion" also comes to refer to an idealized Jerusalem, city of God (eg. Heb 12:22).
Amos uses the word in parallel to "Samaria's hill" 6:1 which may suggest the citadel where the rich and powerful reside. The use of "Zion" in 1:2 parallels "Jerusalem", and this verse reads like a quotation from a hymn and seems to act as a motto for the book.