A pilgrim was a person who undertook a journey for personal reasons. Often associated with a spiritual journey made manifest in a physical voyage.
In the 1820s on Earth, a popular American preacher first referred to the 1621 English non-conformist settlers of Plymouth as "Pilgrims". The Amercian populace came to accept this term.
Spock referenced these 1621 settlers when attempting to describe the mentality of the Horatian system settlers. (TOS novel: Mission to Horatius)