The exercise in which you lie on your side and lift your top leg toward the ceiling is a side leg raise.
Also called a side-lying leg raise, this movement is performed with the body supported on one side while the upper leg abducts away from the midline. The working leg moves upward against gravity, then returns under control. The exercise commonly targets the hip abductors, including the gluteus medius and gluteus minimus, along with muscles such as the tensor fasciae latae.
It is often confused with a clamshell. In a clamshell, the knees remain bent and the feet are usually kept together while the top knee opens. A hip thrust or glute bridge instead involves raising the pelvis, usually from a supine position, rather than lifting one leg sideways.
Good form usually means keeping the torso stable, the hips stacked, and the leg from rotating dramatically outward. Raising the leg only as far as the pelvis stays controlled is more useful than chasing maximum height. Variations include ankle weights, resistance bands, and bent- or straight-knee versions.