"Borderline cases" refers to situations or cases that are on the line between two different outcomes or categories. Synonyms for this phrase include ambiguous, equivocal, uncertain, indeterminate, borderline, dubious, undecided, and unsure. These words all refer to something that is not completely clear-cut or definite, but rather hangs in the balance between two different possibilities. Some other related terms include murky, murky, hazy, fuzzy, and shadowy, which all convey a sense of haziness or obscurity. When dealing with borderline cases, it's important to consider all the available information and to make a decision based on the most reliable evidence at hand.