Which term best describes a lack of quality or quantity required for a purpose?

The term "inadequate" specifically conveys a sense of not meeting the necessary quality or quantity needed for a particular purpose. This word embodies the idea that something does not reach the required level to be effective or sufficient in fulfilling its intended function.

While "deficient" may also imply something is lacking, it often suggests a specific shortcoming rather than a broader sense of inadequacy for a purpose. "Substandard" typically refers to something that falls below an established standard, but it doesn’t solely focus on the quality or quantity aspect. "Insufficient" implies not enough of something, but it might be perceived as more quantitative without the qualitative implications that "inadequate" encompasses.

Thus, "inadequate" is the most fitting term, capturing both the qualitative and quantitative aspects of what is lacking for a specific purpose.