Allegra is the Italian word for "joyful" or "lively" — the musical term allegro shares the root. Lord Byron named his illegitimate daughter Allegra (1817-1822).
Allegra is rare in the U.S. The actress Allegra Versace is a contemporary bearer.
Allegra 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 Allegra reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.