In its most basic terms, slavery is a system in which a human is treated as property and forced to work without compensation.
Slavery has been part of the human experience for most of recorded history. In the 18th and 19th centuries, various conscientious efforts were made to end the institution. While widespread chattel slavery was ended before 1900, most notably in Europe and the Americas, the institution does continue on in the 21st century, whether sanctioned by governments or not.
Slavery was initially unheard of in the uninhabited wilds of Atlantis; however, in time, the French and Spanish settlements began an active trade in African slavery. Additionally, following the Basque discovery of Terranova, copperskins were also forced into slavery. Dissatisfied slaves severely hampered both French and Spanish war efforts against England during the Atlantean theatre of a mid 18th century global conflict. Victor Radcliff's closest military adviser was Blaise, an escaped black slave. At the end of the war, Britain controlled French Atlantis. Slavery was left intact; some English Atlanteans even headed south to become part of the plantation system.
A dozen years after Britain took control of France's Atlantean settlements, English Atlantis, dissatisfied with the increasingly autocratic rule imposed on them by London, rebelled, launching the Atlantean War of Independence. French Atlantis joined in that rebellion from the outset. When the war was over, the settlements had won, forming the new United States of Atlantis. Despite the new country's ideals of equality and freedom, slavery was not addressed by the new government, and the institution continued. Not long after, slavery expanded with the purchase of Spanish Atlantis.
Slaves did not always accept their status, and rebellions took place off and on throughout the remainder of the 18th Century, and well into the 19th Century. Anti-slavery sentiment grew in the northern half of the country, but southern slave-owners grew increasingly defensive of their system. Things reached their climax in 1852, when Frederick Radcliff, a slave and descendant of Victor Radcliff launched what became the Atlantean Servile Insurrection, a civil war that finally brought an end to slavery in Atlantis.
Santo Tomás, a French colony south of Atlantis, saw its slaves rise up and bloodily overthrow their oppressors in the late 18th century. However, a nearby Spanish colony continued to maintain a slave economy even after the Atlantean Insurrection, and so did the Empire of Huy-Braseal in southern Terranova. Life for slaves in these nations was said to be even worse than it had been in Atlantis.
Agrippan Rome, having never modernized, still used slavery as a fundamental part of its economy and society. The Solters family found slavery repulsive, but also found themselves heavily burdened by the sheer number of domestic tasks they had to accomplish alone. Crosstime Traffic regulations specifically forbid agents from owning slaves; there had been quite the scandal a few years in the past.
The slavery scandal that rocked Crosstime Traffic was discovered by Annette Klein, a teenager from the home timeline who was captured by slavers and sold into slavery in Madrid in a medieval Europeanalternate that never recovered from the Great Black Deaths. The Madrid slavers in turn sold her to an illegal Crosstime slave outpost in an uncharted alternate, where people owned and abused slaves freely, against government regulations. This consortium got extra income from the bizarre industry of "slave tourism" wherein idle, wealthy people of the home timeline paid to have themselves enslaved temporarily as a thrilling adventure. Annette was eventually able to escape to the home timeline and alert Crosstime to this abuse of power. All major ringleaders were sent away for lengthy prison terms.
When the ruling class of the world's first sentient species - the reptilian Liskash - first encountered a second sentient species - the mammalian Mrem, one of their first acts was to capture some of the felines and bind them to slavery in the domains of Liskash noblemen.