What does 线索 (xiànsuǒ) mean in English?

Card 2818HSK Level 42818/3571
CARD2818
PROGRESS79%
线索
線索
xiànsuǒ
clue; key(Noun)
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Reveal: Click/Space/Enter/↑↓/w/s/j/k•Navigate: ←→/h/l/a/d