A commonplace book, but specifically for musical ideas in the context I encountered it.
An added bonus given to you by a merchant when you buy something. By extension, an added bonus.
In our grammar book we are using in my Structure of Spanish class (King & Suñer), they explain the contrast between lexical categories which are closed (prepositions, articles, pronouns, conjunctions, etc...) and those that are open. Every speaker of the language knows all the pronouns and articles, but not all the nouns, adjectives, verbs, and adverbs. Also, we can easily add new nouns, borrowing from Italian or Cajun French as in these two examples, but it is really hard to introduce into the language a new definite article or pronoun. People are resistant about trying to introduce new pronouns like zir, but they don't get mad if I tell them about the word zibaldone.
Other sources talk about function words and content words. To understand a function word, one studies grammar; to understand a content, word, we look it up in the dictionary.