No matter what you call it, it is an essential ingredient for alfajores. Image on Flickr by user Kim Love (CC BY-SA 2.0).

In Spanish it's known as dulce de leche, manjarblanco, manjar, arequipe and cajeta. All of those names refer to the same traditional ingredient — a type of caramelized milk that's indispensable in homemade and industrial Latin American confectionery.

Every year on October 11, the sweet is celebrated on World Dulce de Leche Day (using the name it is most widely known as). There are many versions of its origin story, and the most popular one places it in Argentina:

[…] el 11 de octubre [de 1829], [el militar y político argentino] Juan Manuel de Rosas y su oponente político, [el militar y político argentino] Juan Lavalle, se reunían para firmar un acuerdo de paz en la estancia “La Caledonia” en Cañuelas.
[…] una criada estaba en la estancia preparando la lechada (leche caliente con azúcar) con la que Rosas tomaba su tradicional mate. […] llegó el General Lavalle tan cansado que se tiró a dormir una siesta en la hamaca que pertenecía al dueño de casa. Frente a ese panorama, la criada salió desesperada a buscar a la guardia y olvidó la mezcla cocinándose en la olla. Cuando regresó a su puesto, ya se había convertido en una pasta espesa y de color marrón.
Con miedo, le confesó a Rosas lo que había pasado con su lechada [quien] lejos de enojarse, probó lo que había en el recipiente y, tanto le gustó, que se lo convidó a Lavalle.

[…] on October 11 [1829], [Argentinian politician and army officer] Juan Manuel de Rosas and his political rival, [Argentinian politician and army officer] Juan Lavalle, got together to sign a peace agreement in La Caledonia farm in Cañuelas.

[…] a maid was at the farm preparing lechada (hot milk with sugar) that Rosas used to drink his traditional mate with. […] General Lavalle was so tired when he arrived that he fell asleep on the hammock that belonged to the farm's owner. When the maid saw this, she ran in shock to tell the guards and completely forgot the mixture that she was preparing. When she got back, it was already a thick, brown paste.

Scared, she confessed to Rosas what had happened with his lechada [and] instead of getting mad, he tried what was in the pot and liked it so much that he shared it with Lavalle.

Other stories date the confection to an earlier time:

Otras leyendas aseguran que en Chile el Libertador General San Martín lo degustaba en su estadía tras su gesta Libertadora y que pronto se trasladó a Mendoza y luego a Buenos Aires.
En Francia dicen que el hallazgo (accidental) ocurrió en la campaña Napoleónica, los veteranos enrolados con el corso, recibían a diario una ración de leche azucarada caliente, un cocinero que estaba preparando la misma, a quien asistió el fragor de la batalla, abandono la olla, con la leche y el azúcar en la hornalla encendida.
Al hervir se transformo en Confiture de Lait (Dulce de Leche), y el cocinero entró en la historia francesa, como su inventor en el año 1815.

Other legends say that in Chile, the Liberator General San Martín used to savor it during his stay after the war for freedom, and [it] soon was taken to Mendoza and then to Buenos Aires.
In France, the story says that the (accidental) finding happened during Napoleon's campaigns, when recruited veterans used to get a daily share of hot sugary milk and one day a cook left the pot unattended with the milk and sugar on the hot burner.

After boiling, it became confiture de lait (dulce de leche), and the cook went down into French history as its inventor in 1815.

In any case, each October 11 on World Dulce de Leche Day, this delicacy is honored with different candy and dessert fairs, where the public can taste and buy all kind of sweets. Cooks also share their secrets with the public.

Leading up to the occasion, people have been talking about the milk caramel on Twitter, using all the different names this sweet ingredient is known by:

Arequipe, dulce de leche, majar blanco, mira su origen y receta original ¿Qué tanto te gusta el #ManjarBlanco ? https://t.co/g8qbaXolth pic.twitter.com/Hk4tpk3RtY

— Bésame FM (@BesameFM) 27 de septiembre de 2016

Arequipe, dulce de leche, manjar blanco, take a look at its origin and original recipe. How much do you like it?

Yo que creía que Guadalajara de Buga, en el Valle del Cauca…
Manjar, arequipe… ¡Manjarblanco! https://t.co/SsLSn8yLsT

— Ⓜarino Giraldo (@MarinoGiraldo) 26 de septiembre de 2016

I used to think that Guadalajara de Bugue, in Cauca Valley… Manjar, arequipe… manjarblanco!

Me recibí de GORDO. Le entré al pote de dulce de leche

— Thomas Gelmi (@ThoGelmi) 27 de septiembre de 2016

I graduated as a FAT MAN. I had a jar of dulce de leche.

Mi amor por los churros con arequipe es demasiado puro y sincero.

— Nanny (@Nannyvelasquez) 27 de septiembre de 2016

My love for churros with arequipe is so pure and sincere.

In the end, it doesn't matter what you call it or where it comes from. What matters is that traditional desserts and candy would be unimaginable without this sweet ingredient.