A double helix is the structure of a molecule of DNA, which takes the form of two long strands twisted together. It wasn't until the 1950s that scientists began to understand the double helix shape of DNA.
The work of many scientists went into uncovering DNA's double helix structure (including Rosalind Franklin, James Watson, and Francis Crick), and it's still considered to be one of the most important scientific discoveries ever. The term comes from the Latin helix, or "spiral," and the fact that these molecules look like ladders twisted into a spiral. The spiral-shaped Double Helix Nebula at the center of our galaxy was named for its similarity to a DNA molecule.