major power; principal countries
Refers to nations with significant political, economic, or military influence, often used in international relations contexts.
The summit of major powers will be held next month.
Many principal countries are participating in this treaty.
先進国 refers specifically to developed countries based on economic and technological advancement, whereas 主要国 is broader and can include emerging powers.
Compound of 主要 (shuyō, 'main, principal') and 国 (koku, 'country'). The reading is on'yomi.