This is the 3867th most frequent Spanish word.
terceras
"Third parties" or "thirds" (depending on context).
Here, 'terceras' modifies 'personas' to indicate 'third-party individuals'.
Ella siempre evita involucrarse en discusiones de terceras personas.
She always avoids getting involved in third-party discussions.
In this sentence, 'tercer' (not 'terceras') is used as an ordinal adjective meaning 'third', showing different usage.
En la carrera de matemáticas, los alumnos de tercer año usan calculadoras.
In the mathematics course, third-year students use calculators.
Here, 'terceras' is used as an adjective to describe 'fuentes', indicating external sources.