Well, so what do you call a whole week of constant 34℃ during the day, but with clouds in the sky and even a brief (but intense) rain once? That's what we had last week in Poland.

Agreed. It's a heatwave when this kind of weather (hot/dry, hot/normal, hot/humid) persists. Sometimes in combinations of these conditions.

Something that is a heat wave in one place might just be called 'weather' in another place. If New Delhi got our "regular winter weather", it would be called way more than a "cold spell", while 2 weeks of New Delhi weather here is called a heat wave.