Avenseguim is the incantation of a spell that converts a target object into a tracking device (CG48).
References from the canon
Newton Scamander used this Incantation for a unnamed spell in 1927 Paris while searching for Tina Goldstein. He used it to track the origins of a feather from the hat of Yusuf Kama back to its owner, his only clue to her whereabouts (CG48).
The incantation Avenseguim is possibly derived from avens, a Latin adjective for "eager" or "craving", and seguir, the Spanish and Portuguese verb meaning "to follow". Together, Avenseguim can be interpreted as "to eagerly follow".
The incantation is incorrectly spelled as "Avensegium" in the companion books, "Fantastic Beasts: The Crimes of Grindelwald: Magical Movie Handbook" and "Harry Potter & Fantastic Beasts: A Spellbinding Guide to the Films".