Coriander or cilantro?

I did not remember how to say "cilantro" in English, so I asked Moni. She told me "coriander". I remembered looking the word up in the dictionary, and it was the same answer: cilantro (Spanish) = coriander (English).

I went to see Mark and Brigid who are making guacamole for tonight's dinner with my bunch of "coriander".

"Hey, Mark, do you need some coriander for your guacamole?" I asked.

"I don't know" Mark said, "ask Brigid".

Brigid was chopping in the balcony while Mark watched over their kids.

I showed my bunch of coriander to Brigid asking the same question, and she replied:
"Oh, that is not coriander, that's cilantro!"

I was amazed, our American friends who live in Brooklyn use the same Spanish word! And the word "coriander" is not even familiar to them.

By the way, is "cilantro" a Spanish word? I don't know anymore!