Two definitions from Dictionary.com:
1. a particular occupation, business, or profession; calling.
2. a strong impulse or inclination to follow a particular activity or career.
Words from others on the topic:
“Each man has his own vocation; his talent is his call. There is one direction in which all space is open to him.”
-Ralph Waldo Emerson
“An unfulfilled vocation drains the color from a man's entire existence.” -Honore de Balzac
“Vocation is the spine of life.” -Friedrich Nietzsche
“The artist vocation is to send light into the human heart.” -George Sand
“The vocation of every man and woman is to serve other people.” -Leo Nikolaevich Tolstoy
“If each one does their duty as an individual and if each one works in their own proper vocation, it will be right with the whole.” -Johann Wolfgang von Goethe
"Vocations which we wanted to pursue, but didn't, bleed, like colors, on the whole of our existence." -Honore de Balzac
"To hunger for use and to go unused is the worst hunger of all." -Lyndon B. Johnson