Cilantro vs. coriandro
While both cilantro and coriandro refer to the plant *Coriandrum sativum*, they are not widely interchangeable in daily Spanish. Cilantro is the dominant term for the fresh herb, while coriandro is generally restricted to the dried seeds, scientific contexts, or bad translations.
Cilantro
A2This is the standard word used in Spain and Latin America for the fresh green leaves of the plant. If you are cooking a dish that requires the green herb, cilantro is the correct choice.
Por favor, pica un poco de cilantro fresco para la sopa.
(Please chop some fresh cilantro for the soup.)
A algunas personas el cilantro les sabe a jabón.
(To some people, cilantro tastes like soap.)
Los tacos al pastor siempre llevan piña, cebolla y cilantro.
(Al pastor tacos always have pineapple, onion, and cilantro.)
Olvidé comprar el manojo de cilantro en el mercado.
(I forgot to buy the bunch of cilantro at the market.)
Coriandro
B2This word is much less common and is primarily used to refer to the dried seeds (used as a spice) or the essential oil. It is also used in botanical contexts, but rarely for the fresh leaves in a kitchen setting.
La receta pide una cucharadita de semillas de coriandro molidas.
(The recipe calls for a teaspoon of ground coriander seeds.)
El coriandro tiene un aroma cítrico muy agradable.
(Coriander has a very pleasant citrus aroma.)
En la perfumería se usa mucho el aceite de coriandro.
(Coriander oil is used a lot in perfumery.)
El nombre científico de la planta es coriandro, pero todos la llamamos cilantro.
(The scientific name of the plant is coriander, but we all call it cilantro.)
Summary
Use cilantro whenever you refer to the fresh green leaves used in guacamole, salsa, or salads. Use coriandro primarily when referring to the dried seeds used as a spice, though many native speakers simply say semillas de cilantro (cilantro seeds) for that as well.