They are called Placeholder Names

Placeholder names are words that can refer to objects or people whose names are temporarily forgotten, irrelevant, or unknown in the context in which they are being discussed.

Consider Alice and Bob

Alice and Bob are two commonly used placeholder names. They are used for archetypal characters in fields such as cryptography, game theory and physics.1 The names are used for convenience; for example, "Alice sends a message to Bob encrypted with his public key" is easier to follow than "Party A sends a message to Party B encrypted by Party B's public key." Following the alphabet, the specific names have evolved into common parlance within these fields—helping technical topics to be explained in a more understandable fashion.

They are called Placeholder Names

Placeholder names are words that can refer to objects or people whose names are temporarily forgotten, irrelevant, or unknown in the context in which they are being discussed.

Consider Alice and Bob

Alice and Bob are two commonly used placeholder names. They are used for archetypal characters in fields such as cryptography, game theory and physics.1 The names are used for convenience; for example, "Alice sends a message to Bob encrypted with his public key" is easier to follow than "Party A sends a message to Party B encrypted by Party B's public key." Following the alphabet, the specific names have evolved into common parlance within these fields—helping technical topics to be explained in a more understandable fashion.

[Wikipedia]