1080i refers to a digital video signal of 1080 lines in interlaced video. Normally the 1920 pixel data is ignored. So the definition of the complete signal is 1920 x 1080i, a signal with a binary regime of 1.5 Gbps.
1080p refers to a digital video signal of 1080 lines in progressive video. Normally the 1920 pixel data is ignored. So the definition of the complete signal is 1920 x 1080p, signal with a binary regime of 3 Gbps.