Use the word amigo to talk about a friend in a casual, colloquial way. When you run into an old pal you haven't seen in a while, you might call out, "Hey, amigo! Nice to see you!" It's most common to use this word in Spanish-speaking parts of the US, though most people will know exactly what you mean. It is, in fact, the Spanish word for "friend," from the Latin amicus, also "friend."