1. Sliding Window and Casement Window:

No, a sliding window and a casement window do not have the same meaning at all, as they refer to two very different opening mechanisms.

2. Casement Window (or French Window)
The casement window, often called a "French window," is the most traditional type of window.

It opens by pivoting on a vertical axis, like a door, generally toward the interior of the room. It can have one or more sashes (the "casements") that open independently.
It allows for full opening, facilitating rapid ventilation and window cleaning. It generally offers very good thermal and sound insulation.
Opening the sashes takes up space inside the room, which can be a disadvantage in tight spaces.
3. Sliding Window (or Bay Window)
The sliding window, often associated with bay windows, operates differently.

Its sashes slide horizontally one behind the other on rails, or, in the case of a pocket window, are inserted into the thickness of the wall.
You save significant space because it doesn't encroach on the interior space. It allows for large glass surfaces, providing maximum brightness and an unobstructed view.
However, it doesn't allow full opening (the sashes overlap), which can reduce maximum ventilation compared to a casement window. The airtightness may be slightly lower on lower-end models.
In short, the main difference lies in the opening method: casement windows open by pivoting, while sliding windows open by sliding.