Is "ought" a word, or just slang?

T Baggin asks, "Is ought an actual word in the English language, or just slang?"

Thank you for this question.  

Ought is definitely an English word. It is a modal verb that is almost always followed by to + the infinitive form of a verb, as in these examples:

1. 1. They ought to be here by now.
   2. There ought to be a gas station on the way.
   3. He ought to have known better. 
   4. You ought to get some rest. 

Ought to has the same meaning as the modal should, and it is used in the same ways, but ought to is less common and more formal than should.

Modal verbs are verbs that are not conjugated. They are used to signal things like obligations, expectations, advice, and suggestions. They have only one form, which does not change. The verb after the modal changes to indicate past tense, as shown in sentence 3 above: He ought to have known better.

By the way, even if ought were slang, which it isn't, it would still be a word, and it would still be listed in dictionaries. Slang words serve an important function in informal speech.

I hope this helps!