Eau gazeuse vs. eau pétillante

'Eau gazeuse' and 'eau pétillante' are French terms commonly used to refer to carbonated water. While they are often used interchangeably, some subtle differences in usage and connotation may exist, reflecting the level of carbonation or context in which they're served.

Eau gazeuse

This term is generally used to refer to any type of carbonated water including mineral water with natural gas or plain water that has been artificially carbonated. It's equivalent to 'sparkling water' in English.
Pouvez-vous me passer une bouteille d'eau gazeuse, s'il vous plaît ?
(Could you please pass me a bottle of sparkling water?)
Je préfère l'eau gazeuse à l'eau plate lors des repas.
(I prefer sparkling water over still water during meals.)

Eau pétillante

Similar to 'eau gazeuse', 'eau pétillante' refers to sparkling water but often implies a slightly stronger emphasis on the fizziness of the water. It may be preferred when distinguishing from lightly carbonated waters.
Cette eau pétillante a vraiment beaucoup de bulles !
(This sparkling water really has a lot of bubbles!)
Pour ma part, je choisis toujours l'eau pétillante au restaurant.
(For my part, I always choose the fizzy water at the restaurant.)

Summary

In summary, while 'eau gazeuse' and 'eau pétillante' can both be used to describe carbonated water in French, 'eau gazeuse' is a broader term that encompasses all types of sparkling waters. In contrast, 'eau pétillante' sometimes suggests a higher level of carbonation and is often used when the bubbly nature of the water is a focal point. In practice, the two terms are largely interchangeable and understood by French speakers without significant distinction.