Pack Year

Definition:
Pack year is a term used to describe the amount a person has smoked over time. One pack year is defined as 20 manufactured cigarettes (1 pack) smoked per day for one year.

Example:

I smoked 1 1/2 packs per day for 26 years, which works out to 39 pack years:

1 pack per day x 26 years = 26 pack years
1/2 pack per day x 26 years = 13 pack years
26 pack years + 13 pack years = 39 pack years

_{Source:
National Cancer Institute Dictionary of Cancer Terms}