countries around the world; all nations
Refers to the countries of the world collectively, often used in contexts like international news, global surveys, or worldwide events.
世界各国から観光客が訪れる。
Tourists visit from countries all over the world.
そのニュースは世界各国で報道された。
The news was reported in countries around the world.
Tourists from countries around the world gather for this event.
The survey is being conducted in countries around the world.
The technology spread to countries around the world.
万国 is an old-fashioned, formal word for 'all nations' seen in fixed terms like 万国博覧会, while 世界各国 is the standard modern expression.
世界各国 specifically means 'countries around the world', while 世界各地 refers more broadly to places or regions, not limited to national units.