The quality/ability/extent of being extensible.
Extensibility is a system quality attribute that represents the system's capacity to extend its functionality or accommodate new features and requirements efficiently. It is a non-functional requirement that describes the ease with which the system can incorporate new functionality without disrupting the existing system's structure or requiring significant changes.
Extensibility is a cross-functional constraint as it impacts various stakeholders involved in the system's development, such as developers, testers, end-users, business analysts, and system administrators. It influences the system's overall architecture, design, and implementation, and affects the system's maintainability, scalability, and adaptability.
Define extensible: In the context of computers and software, extensible means that a system or program is designed in a way that allows it to be easily enhanced or modified with new features or functionalities without requiring significant changes to the underlying code or architecture. An extensible system typically provides mechanisms such as APIs (application programming interfaces), plugins, or scripts that developers can use to extend its capabilities and integrate it with other systems, tools, or platforms. This flexibility and adaptability make extensible systems and software ideal for various applications, from complex enterprise systems to consumer applications and mobile apps.
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Wikipedia: Extensibility: TODO
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Dictionary: extend: Extend. to stretch out; draw out to the full length. to stretch, draw, or arrange in a given direction, or so as to reach a particular point.