A necessary contingent apparatus that carries the essential elements of one thing back and forth from something else? 

GPT (a tool) says:
A "tool" is a device or implement, typically handheld, used to carry out a particular function. In a broader sense, a tool can be any item or software used to achieve a specific goal or perform a specific task. The definition can be expanded to various contexts:

• Physical Tools: These are traditional tools like hammers, screwdrivers, or pliers, used for physical tasks such as construction, repair, or crafting. They are typically designed to make work easier and more efficient by providing mechanical advantage or precision.

• Software Tools: In the digital realm, a tool refers to a software application or program that is used to perform specific digital tasks. Examples include word processors for writing, graphic design software for creating digital art, or data analysis programs for processing information.

• Scientific and Technical Tools: In science and technology, tools can refer to instruments or devices used to perform experiments, measurements, or other specialized tasks. Examples include microscopes in biology, telescopes in astronomy, or software tools for computational analysis.

• Conceptual Tools: In a more abstract sense, a tool can also refer to a conceptual or methodological approach used to solve problems or achieve objectives. For example, in management, a tool might be a strategic framework like SWOT analysis; in philosophy, it could be a method of logical reasoning.

• Educational Tools: These are resources or methodologies used to facilitate learning. Examples include textbooks, educational software, interactive whiteboards, and even teaching strategies.

• Artistic Tools: In the arts, tools can refer to anything used to create art, from paintbrushes and canvases to musical instruments and digital art software.

The key aspect of a tool is its utility in facilitating tasks, solving problems, or creating things, whether those tasks are physical, intellectual, or creative in nature.