What does the term "schema-less" indicate in MongoDB?

The term "schema-less" in MongoDB indicates that documents can have varying structures. This characteristic is one of the defining features of MongoDB's flexible data model, allowing developers to store data in a more dynamic and adaptable manner.

In a schema-less system, each document in a collection can have different fields and structures, meaning that developers can evolve their data models over time without the need for complex database migrations. This flexibility is particularly useful in applications where the data evolves frequently, enabling rapid development and iteration.

The other options suggest constraints on data structure that do not align with the schema-less principle of MongoDB. For example, requiring a defined schema or conforming to an existing schema at all times would limit adaptability and flexibility, which is contrary to the core design of MongoDB. Thus, the concept of being schema-less fundamentally allows for diverse and dynamic document structures within a single collection.