One festival. Two names. One Sanskrit root, and a few popular stories about it that aren't quite true.
Whether you grew up calling it Dasara or Dussehra, both names trace back to the same Sanskrit word: दशहरा, daśaharā.
Ten what? Classical Sanskrit dictionaries like Apte and Monier-Williams answer: ten sins. Hold that thought, it's worth coming back to further down.
Sanskrit, Latin, Greek, and English all inherited their word for "ten" from the same shared ancestor language, reconstructed by linguists as Proto-Indo-European *déḱm̥.
December was the 10th month of the early Roman calendar. Dashamukha, "ten-faced," is a name for Ravana — a word built from the very root this article is about.
Neither name is a mispronunciation of the other. Both are regular, separate sound changes applied to the same Sanskrit word as it passed into different modern languages.
The "sh" softens to "s" and the middle "ha" drops out: daśaharā → dasarā.
Hindi drops the short middle vowel instead: daśaharā → daśharā. The "u" spelling is old British transliteration for a short "a," the same convention that gives us "Punjab."
Different sounds, same word, same meaning: "the one that takes away ten."
Here's where it gets interesting: the festival named by this one word is celebrated as two different victories, depending on where in India you are.
Goddess Chamundeshwari slays the buffalo-demon Mahishasura, the demon Mysuru is believed to be named after. The festival has been held since 1610, begun by Raja Wodeyar I, and includes the Jamboo Savari elephant procession, Gombe Habba doll displays, and Ayudha Puja, where tools and vehicles are honoured.
Lord Rama defeats Ravana, the ten-headed king of Lanka, and rescues Sita. Ramlila plays are staged through the nine nights, and giant effigies of Ravana, Kumbhakarna, and Meghnad are set alight at dusk, with fireworks and fairs across towns and cities.
Both land on the same day: Vijayadashami, from vijaya "victory" + daśamī "the tenth lunar day." Spot the daśa hiding in there too.
दस + सिर (das + sir, "ten heads") is not where the word comes from. The real breakdown is दश + हरा (daśa + harā, "takes away ten").
There's no "head" anywhere in the word. The second half is harā, "taking away." Ravana's ten heads make a vivid story, but it's a popular reading, not what the word says, and it can't explain why Karnataka uses the exact same name for a festival that's about Chamundeshwari, not Ravana at all.
Dasara and Dussehra are siblings, not parent and child. Each grew separately from the same Sanskrit दशहरा, daśaharā, through its own regular sound changes (see "So what actually changed," above). And Dasara isn't only southern: Marathi, spoken well north of Karnataka, calls it दसरा too.
That verse isn't about autumn at all. It names the Ganga's festival, the bright tenth day of Jyeṣṭha (May–June), when she is said to have descended to earth. It's still celebrated separately today as Ganga Dussehra. The name daśaharā is older and broader than any one story told about it, which is exactly how it ended up attached to two unrelated autumn victories at once.
One daśa gave you Dasara. Its distant cousin decem gave English decimal and December. That's the whole idea behind Atreemology: learn one root, then watch the real words grow out of it, instead of memorizing a list of unconnected vocabulary.
Start growing your forest, it's free