A Product Backlog is:
Managed by the Product Owner. The Product Owner is the sole person responsible for managing the Product Backlog, which is an emergent, ordered list of what is needed to improve the product. They must ensure that the Product Backlog is transparent, visible, and understood by everyone who needs to work on it.
An inventory of things to be done for the Product. The Product Backlog contains all the features, functions, requirements, enhancements, and fixes that constitute the changes to be made to the product in future releases. The Product Backlog items have a description, an order, an estimate, and a value.
Ordered based on priority, value, dependencies, and risk. The Product Owner orders the Product Backlog items based on various factors that affect their importance and urgency for the product. These may include customer needs, business value, stakeholder feedback, technical dependencies, or market opportunities.
Other options, such as an exhaustive list of upfront approved requirements to be implemented or only visible to the Product Owner and stakeholders, are not valid descriptions of a Product Backlog. They may reflect a misunderstanding of what a Product Backlog is or how Scrum works.
Reference:
[Scrum Guide], page 6, section "Product Owner"
[Scrum Guide], page 11, section "Product Backlog"
[Professional Scrum Product Owner Training], page 7, section "Product Backlog Management"