Is Castilian the same as Spanish?

Yes, for the most part. Castellano (Castilian) and español (Spanish) are two names for the same language. Castellano is common in Spain and in parts of South America, and español is more common in much of Latin America and in English. Sometimes "Castilian" also means the Spanish of central and northern Spain specifically.

Where the name comes from

Spanish began as the language of the medieval Kingdom of Castile, in central Spain. As Castile grew and became the center of the Spanish state, its language spread across the country and later to the Americas. So the language is named after the region it came from: Castilian, castellano.

Why Spain uses "castellano"

Spain has other languages besides Spanish. Catalan, Galician and Basque are official in their regions. Calling the national language castellano distinguishes it from those, and avoids suggesting that the other languages are somehow "not Spanish". It's partly a matter of precision and partly of politics.

In Spain you'll hear both words, but castellano is very common, especially in regions that have their own language.

What about Latin America?

Usage varies:

• Argentina, Chile, Colombia and others often say castellano, especially in schools and formal contexts.
• Mexico mostly says español.
• English speakers nearly always say "Spanish", and "Castilian" sounds formal or old-fashioned.

Neither word is wrong. If you say español, you'll be understood everywhere. If you say castellano, you'll be understood everywhere too.

The other meaning of "Castilian"

When people say "Castilian Spanish" in English, they sometimes mean the accent of central and northern Spain, the one with the "th" sound for z and c before e and i, known as distinción. In that sense it describes one accent, not the whole language.

This is where learners get confused. Is "Castilian" a language, or an accent? Both uses exist. The context usually tells you which is meant. If someone says "I want to learn Castilian", they most likely mean the Spanish of Spain, but you can ask.

Does it matter for learning?

Not much. You don't need to learn a different language. The Spanish you learn from any course is Castilian in the first sense. Whether you pick up the "th" sound and vosotros depends on which region you aim for. See Spain or Latin American Spanish and Mexican vs Spain Spanish.

Whichever name you use, what moves you forward is hearing the language a lot. Full disclosure, I built Langtok for that: short Spanish stories you swipe through, with every word highlighted as it's spoken.

Short version

Same language, two names. Use español if you aren't sure, and castellano if you're in Spain and want to be precise about the difference from Catalan, Galician or Basque.