Definition
Scarce measures a ratio, not an amount: something is scarce when the wanting outruns the having. That is what separates it from 'rare' — gold is rare everywhere, but water is scarce only where thirst exceeds the wells. Economics calls itself the study of scarce resources for exactly this reason: scarcity is the founding condition that forces every choice. And the first thing scarcity changes is never the resource — it is the behavior of the people seeking it.
Examples
Collocations
scarce resources·increasingly scarce·a scarce commodity·scarce supplies·make oneself scarce
Synonyms
in short supply·sparse·meager·insufficient·rare
Antonyms
abundant·plentiful·ample
Word family
scarcity (noun)·scarcely (adverb — 'hardly', a shifted sense)
In TOEFL & IELTS
'The allocation of scarce resources' is the textbook definition of economics, and TOEFL passages quote it almost verbatim. IELTS essays on housing, water, and jobs all reward 'increasingly scarce' and the noun 'scarcity'. Two traps: 'scarcely' means 'hardly' (degree, not supply), and scarce ≠ rare — scarce is always relative to demand, which is the distinction reading questions test.