Self-Sovereign Identity, as a standalone phrase or as an acronym (SSI), is a very confusing, overloaded, ambiguous term whose use should be avoided in all situations. In a conversation, a blog post, an article, or a standards document, it could mean any of the uncontracted terms in the following figure.
 The most recent example of SSI confusion is the Principles of SSI document published by the Sovrin Foundation (https://sovrin.org/principles-of-ssi/). The document makes no attempt to define SSI nor state what SSI is intended to be an acronym for. Principles of SSI has resulted in a lot of confudsion (https://hyperonomy.com/2018/12/18/definition-confuding/) in the decentralized identifier communities as different working groups try to apply the Principles in new domains without first discerning: a) precisely which definition/application of SSI should be used and b) the types or categories of principles being represented in the document. Without first having a clear and precise definition for (a), successfully achieving (b) is almost impossible.
The term Self-Sovereign Identity (or its acronym SSI) could refer to any or all of the uncontracted terms in the above figure – unless an author has taken great care to be clear and precise with respect to every occurrence/usage of the term and/or its acronym.
What is Self-Sovereign Identity (SSI)? …really
To understand, in plain English, what a Self-Sovereign Identity (SSI) is, one must first recognize that “Self-Sovereign” is an adjective applied to the noun “Identity”. The TDW Glossary derivation of Self- Sovereign Identity is depicted in the model below.
Decentralized Identity is derived from (or is a specialization of) the term Digital Identity.
Self-Sovereign Identity, in turn, is derived from (or is a specialization of) the term Decentralized Identity.
In addition, Anonymous Self-Sovereign Identity and Non-anonymous Self-Sovereign Identity are derived from (or are a specialization of) the term Self-Sovereign Identity.
A Digital Identity aggregates:
A Digital Identifier, and
Associated Digital Identity Data.
A Decentralized Identity is a Digital Identity that is Verifiable.
A Decentralized Identity is often persisted in a Verifiable Data Register.
A Self-Sovereign Identity is a Digital Identity whose Digital Identity Owner has the right to exclusively and permanently exert control over the usage of one or more associated Personal Digital Identifiers and, independently, the usage of any associated Personal Identity Data associated with each Personal Digital Identifier.
Because a Self-Sovereign Identity is a specialization of Decentralized Identity, a Self-Sovereign Identity is also Verifiable.
Digital Identity Owner
A Digital Identity Owner is an entity that may be held legally accountable. Digital Identity Owners includes Persons and Organizations but does not include Things. The actual legal accountability of an Identity Owner for any particular action depends on many contextual factors including the laws of the applicable jurisdiction, etc. [Sovrin Glossary]
Anonymous Self-Sovereign Identity
An Anonymous Self-Sovereign Identity is a Self-Sovereign Identity whose Personal Digital Identifier is associated with (or governed by) an Anonymous Digital Identifier Method.
Non-anonymous Self-Sovereign Identity
A Non-anonymous Self-Sovereign Identity is a Self-Sovereign Identity whose Personal Digital Identifier is associated with (or governed by) a Non-anonymous Digital Identifier Method.
Personal Digital Identifiers
A Personal Digital Identifiers is a Digital Identifier associated with a Person as the Subject of the identifier.
Personal Digital Data
Personal Digital Data is Digital Identity Data associated with a Personal Digital Identifier; and hence, a Person as the subject of the identifier.
Anonymous Digital Identifier Method
An Anonymous Digital Identifier Method is a Digital Identifier Method designed to guarantee anonymity and privacy for the Subject of a conforming Digital Identifier.
Non-anonymous Digital Identifier Method
A Non-anonymous Digital Identifier Method is a Digital Identifier Method that does not guarantee anonymity nor privacy for the Subject of a conforming Digital Identifier.
Self-Sovereign Identity Model (SSI Model)
The next most commonly (and generically) used term is Self-Sovereign Identity Model (SSI Model).
The Self-Sovereign Identity Model (SSI Model) is an identity system architecture based on the core principle that Identity Owners have the right to permanently exert control over the usage of one or more of their Personal Digital Identifiers and, independently, the usage of any associated Personal Identity Data. [inspired by the Sovrin Glossary]