Expression: Coup de foudre
Pronunciation: [coo d(eu) foodr(eu)]
Meaning: bolt of lightning, love at first sight
Literal translation: bolt of lightning
Notes: The French expression le coup de foudre has a literal and a figurative meaning, though the latter is a bit more common. Literally, un coup de foudre is a "bolt of lightning" or "thunderbolt."
Figuratively, it indicates "love at first sight," which I think makes perfect sense, as that feeling is like a huge a shock to the system.
The two verbs you can use with the figurative meaning give it slightly different nuances:
être le coup de foudre - to be love at first sight
Quand je l'ai vu, ça a été le coup de foudre.
When I saw it/him, it was love at first sight.
avoir le coup de foudre (pour) - to fall in love (with) at first sight
J'ai eu le coup de foudre pour Thomas / pour Paris.
I fell in love with Thomas / Paris at first sight.
To fall in love (not necessarily at first sight):
tomber amoureux (de) [not "tomber en amour avec"]
avoir un coup de cœur (pour)
(literary) s'éprendre (de)
(informal) flasher (sur)
To become infatuated (with):
(formal) s'enticher (de)
(informal) se toquer (de)