Lydia comes from the Greek Lydía, the name of an ancient kingdom in western Anatolia (modern Turkey). The biblical Lydia of Thyatira was a wealthy merchant of purple cloth and the first European convert to Christianity (Acts 16).
Lydia peaked at #74 in the U.S. in 1880 and has remained continuously used. Today it sits in the U.S. top 100 and has risen steadily for decades. The character Lydia Bennet in Pride and Prejudice is the most famous fictional bearer.
Lydia 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 Lydia reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.