Coincidencia vs. casualidad
The Spanish words 'coincidencia' and 'casualidad' both translate to 'coincidence', yet, they have slightly different connotations when used in conversation or writing.
Coincidencia
'Coincidencia' refers to the occurrence of two or more events at the same time by accident.
Es una coincidencia que nos encontremos aquí.
(It's a coincidence that we meet here.)
La coincidencia de los eventos fue sorprendente.
(The coincidence of the events was surprising.)
Casualidad
'Casualidad' also means an unexpected, random event or fact that wasn't planned. However, it carries a stronger sense of 'chance' or 'luck' than coincidencia.
Fue casualidad que lleváramos la misma camiseta ese día.
(It was a coincidence that we wore the same shirt that day.)
Por casualidad, encontré el libro que buscaba en la última librería que visité.
(By chance, I found the book I was looking for in the last bookstore I visited.)
Summary
While both 'casualidad' and 'coincidencia' involve unexpected situations or occurrences, 'coincidencia' tends to refer to accidents while 'casualidad' carries a stronger sense of chance or luck. These subtleties can add layers of meaning in communication.