How can I define a property type as being a list (list, set, array, collection) of string in my YAML Swagger definition

For a list of strings, you can describe as follows:

      type: array
      items:
        type: string

Ref: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#schemaObject

Example:

• https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml#L93-L100 (OpenAPI v2)
• https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/resources/3_0/petstore.yaml#L71-L78 (OpenAPI v3)