Tercero vs. tercio
While both tercio and tercero are related to the number three in Spanish, they are not interchangeable and have distinct meanings and grammatical functions. Tercero is an ordinal number indicating position, while tercio is a noun referring to a fraction or portion.
Tercero
A2Tercero is an ordinal number that means third. It is an adjective used to indicate the position or order of something in a sequence. It changes for gender to tercera for feminine nouns, and it shortens to tercer when placed directly before a masculine singular noun.
Vivimos en el tercer piso del edificio.
(We live on the third floor of the building.)
Ella fue la tercera candidata en la entrevista.
(She was the third candidate in the interview.)
Mi equipo quedó tercero en el campeonato.
(My team finished third in the championship.)
Es la tercera vez que te llamo hoy.
(It is the third time I have called you today.)
El tercer capítulo del libro es el más interesante.
(The third chapter of the book is the most interesting.)
Tercio
B1Tercio is a noun that means a third or one-third. It refers to one of three equal parts of a whole. It can also refer to a specific quantity, like a standard 33cl bottle of beer, or have historical meanings, such as a Spanish infantry unit from the 16th and 17th centuries.
Un tercio de los estudiantes aprobó el examen.
(A third of the students passed the exam.)
Por favor, tráeme un tercio de cerveza.
(Please, bring me a third of a liter bottle of beer.)
Gastó un tercio de su sueldo en el alquiler.
(He spent a third of his salary on rent.)
Corta el pastel en tres tercios iguales.
(Cut the cake into three equal thirds.)
Los tercios españoles eran temidos en toda Europa.
(The Spanish tercios were feared throughout Europe.)
Summary
In essence, the main difference is grammatical and semantic. Use tercero (along with its variations tercera and tercer) as an adjective to establish order or sequence, meaning third in a series (e.g., the third car). Use the noun tercio to refer to a fractional part of a whole, meaning one-third of something (e.g., a third of the population).