What is Meant by Permissionless Blockchains?

The term permissionless has established itself within
the context of blockchain and distributed ledger research to
characterize protocols and systems that exhibit similar properties
to Bitcoin. However, the notion of what is meant by permissionlessness is often vague or left implicit within the various literature,
rendering it imprecise and hard to compare. We hereby shed
light onto this topic by revising research that either incorporates
or defines the term permissionless and systematically expose
the properties and characteristics that its utilization intends to
capture. Based on this review, we highlight current shortcomings
and blind spots within the available definitions. In particular,
the ability to freely perform transactions between users is often
not adequately incorporated and different actor roles are left
unspecified. Furthermore, the topics of privacy and governance
appear to be largely overlooked.