Tercero vs. tercer
The Spanish words tercero and tercer both translate to third in English, but each word is used in specific grammatical contexts.
Tercero
A2Tercero is an ordinal number used to indicate position or ranking. It stands for third.
Vivo en el piso tercero.
(I live on the third floor.)
Es el tercero en la lista de espera.
(He is the third on the waiting list.)
Tercer
A2Tercer, another form of specifying third, is used before singular masculine nouns and it's also called apocopated form.
Es el tercer año que estudio español.
(It's the third year I've been studying Spanish.)
Este es mi tercer coche.
(This is my third car.)
Ese es su tercer intento.
(That is his third attempt.)
Summary
Tercero and tercer both express the concept of third in Spanish. The difference lies in their use: tercero, as a full form, is normally utilized independently or after a noun, whereas tercer serves as an apocopated form being placed before singular masculine nouns.