Hidden Level
From Pokemon Go Wiki
Each Pokemon has a Hidden Level. It is the white semicircle that fills up as you Power Up the Pokemon. In addition to adding some CP, each Power Up increases level by 1/2. When the white arc reaches the end, it will say that you need to increase the trainer's level. A Pokemon's Hidden Level cannot exceed your trainer's level + 1.5.
Formula for calculating your Pokemon's current Hidden Level:
- Hidden Level = CP / CP per Power Up / 2
