What term best defines something that lacks support or justification?

The term that best defines something that lacks support or justification is "invalid." In a legal or logical context, an argument or claim is considered invalid if it does not adhere to the necessary criteria or standards for justification. Essentially, an invalid argument is one that fails to establish its claims based on evidence or sound reasoning, meaning it cannot be relied upon or accepted as valid within a particular framework. This reflects the absence of logical consistency or factual support that would substantiate the assertion being made.

While terms like "ineffective," "unsupported," and "unreasonable" also imply a lack of foundation in different contexts, they do not encapsulate the specific legal or logical implication that "invalid" carries. "Ineffective" suggests a lack of efficiency or success, "unsupported" highlights a general absence of backing, and "unreasonable" refers to a lack of rationality. In contrast, calling something "invalid" directly denotes a formal failure to meet the required standards of evidence or justification.