A rare, literary adjectival noun meaning something is uncommon or extraordinary. Not used in everyday conversation; more common synonyms like 珍しい or まれ are preferred.
This specimen is a rare one.
His talent is unusual.
Rare kanji spelling; the word itself is uncommon. 珍希 is an alternative rare spelling.
Alternative rare kanji spelling.
The kanji 珍 (rare, precious) and 稀 (rare, uncommon) combine to reinforce the meaning of rarity. The exact historical derivation is uncertain; it is a Sino-Japanese compound (kango).