Under what name was Waterloo known in France at the time?

In France at the time, Waterloo was known as the Battle of Mont-Saint-Jean.

The main fighting took place around the ridge and hamlet of Mont-Saint-Jean, south of the village of Waterloo in what was then the United Kingdom of the Netherlands. French usage therefore associated the battle with Mont-Saint-Jean rather than Waterloo. The name Waterloo became dominant internationally because the victorious Duke of Wellington issued his official dispatch from the village of Waterloo, where his headquarters were located.

Other armies used different names. Prussian accounts often called it the Battle of La Belle Alliance, after an inn near the battlefield where Wellington and Blücher met after the French defeat. “Waterloo” ultimately prevailed in English and much of wider historical writing, even though the battlefield itself lies mainly in the present-day municipalities of Braine-l’Alleud and Lasne, not in the town of Waterloo proper.

The name Mont-Saint-Jean can be confused with nearby battle sites from the same campaign, such as Ligny and Quatre Bras. Those were separate engagements fought on 16 June 1815, two days before Waterloo.