We use the term attributes to represent the characteristics of an entity. For example, first name, last name, and email address are the attributes of User entities. And, for RDBMS, attributes will be converted into the columns of a table or, for NoSQL, such as MongoDB, they will be converted into the fields of documents. Among the attributes of an entity, there needs to be at least one that can be used to uniquely identify that entity. Take the User entities as an example; the email address of a user can be used to identify that user. Usually, there can be several such attributes.