High-Definition
HD Resolutions are written 720p,
1080i, etc.
The numeric part designates the vertical line-count (i.e. the pixel height,
or rectangle small-side with standard horizontal displays).
The last letter designates the scanning method, to show the image:
-
p: progressive scan.
Single-pass capture. Prevents artifacts, but demands a higher band-width.
-
i: interlaced scan.
Two time-shifted passes. Prone to artifacts but more economical on band-width.
The trade-off is a compromise, both may be preferred depending on needs.
Regardless 1080i and 1080p have the same pixel counts,
this holds in all HD resolutions.
In the rest of the page, they are used interchangeably.