Yes, I understand why "defines" is correct in this context. The word "Baskin-Robbins" may appear to be plural, but it is actually a singular noun because it refers to the name of a single company. Therefore, when using the verb "define" in present tense with this noun, we need to add the third-person-singular ending "-s" to the verb.
To verify this, you can consult a reliable grammar resource such as the one you mentioned (http://grammar.ccc.commnet.edu/grammar/verbs.htm#tense), which provides information on verb tense and usage.
In the case of the sentence "Baskin-Robbins defines a new flavor every month," "defines" is the correct form of the verb "define" because it agrees with the subject "Baskin-Robbins." The use of the third-person-singular ending "-s" indicates that the subject is singular.
I hope this explanation helps! Feel free to ask if you have any further questions.