What's meaning of the sentence "We denote the reprojected point cloud as Pk" [closed]

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Especially "reprojected point cloud"

It comes from LOAM: Lidar Odometry and Mapping in Real-time V-B the first paragraph:

The odometry algorithm estimates motion of the lidar within a sweep. Let t k be the starting time of a sweep k. At the end of each sweep, the point cloud perceived during the sweep, P k , is reprojected to time stamp t k+1 , illustrated in Fig. 6. We denote the reprojected point cloud as P ̄ k . During the next sweep k +1, P ̄ k is used together with the newly received point
cloud, P k+1 , to estimate the motion of the lidar.