In the 1632 series "up-timer" refers to any 20th century American from Grantville, West Virginia. "Up-time" is also an adjective referring to 20th century devices, techniques, customs, etc., and a noun referring to the OTL.
Up-timers generally come from one of three groups. First, and by far the most numerous, are the pre-Ring of Fire inhabitants of Grantville who were in town when the Ring fell. The second group consists of those who were in Grantville as guests at the wedding of Tom Simpson and Rita Stearns, or at the 40th- anniversary party of Vince and Carla Masaniello. There were also a few people who were in town that day for reasons of their own.