A useful way to hold this phrase is to picture a Cold War map with the countries sorted into numbered groups: one, the side that stood with the United States and Western Europe; two, the Soviet side; three, everyone who joined neither. The numbers were about which side a country was on. Because most of the rich industrial states happened to sit in group one, people started using the First World to mean simply the wealthy countries — and that is the meaning you meet far more often today, usually with a poorer group set against it.
One branch
編號的那組世界
兩個意思都從同一張編號地圖出來：一個看立場，一個看財富。
noun · C1
In writing about the Cold War, the group of countries that stood with the United States and Western Europe, as against the Soviet side. This is the older, political use of the phrase, and you now meet it mostly in history books.
第一世界（冷戰時期西方陣營）