Wasabi is the green paste served with sushi and sashimi. It is hot, but hot in a completely different way from chilli: chilli burns your tongue and the heat stays, while wasabi shoots straight up into your nose and then disappears in a few seconds. People sometimes say it "hits your nose" or "goes up your nose", and that is the standard way to describe it in English.

The word is uncountable, like salt or sugar — "a bit of wasabi", "too much wasabi", never "a wasabi" or "two wasabis". It also works in front of another noun: wasabi peas, wasabi mayonnaise, wasabi-flavoured crisps.

Here is the "though actually" part. Real wasabi comes from the root of a plant that is hard to grow and very expensive. Most of what is served in restaurants outside Japan — and honestly in plenty of places inside Japan too — is horseradish coloured green, with mustard added. It tastes similar enough that few people notice. So if an English speaker says "that's not real wasabi", this is what they mean; they are not complaining about the taste, just about what it is made from.

The word came into English from Japanese in the twentieth century, along with sushi itself, and by now nobody thinks of it as a foreign word.