Stands for "Lines Per Inch." LPI is used to measure the resolution of images printed in halftones. Because halftone images are printed as a series of dots, the higher the LPI number, the more dense the dots can be, resulting in a finer resolution.
Newspapers are typically printed in a resolution of 85 lpi, while magazines may use 133 lpi or higher. Because the naked eye can distinguish halftone dots up to about 120 lpi, you are more likely to notice the dots in newspaper print than in magazines. Of course, if you look closely enough, you may be able to see the dots in images printed in 150 lpi or more. But, in normal viewing, it is natural to see the dots as a continuous image even at 85 lpi.