Lucy is the English form of Lucia — from the Latin lux, meaning "light." Saint Lucy of Syracuse (283–304 CE) is one of the most venerated early Christian martyrs; her feast day (December 13) is celebrated in Scandinavia with candle crowns and processions.
Lucy has been continuously in use in English since the medieval period. It dropped from popularity in the mid-20th century but has returned strongly since the 2000s. Today it sits in the U.S. top 45.
Lucy does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Lucy reduce to 7, The Seeker. This is a traditional interpretive system, not a factual claim about the name.