the ability and tendency to determine what should be done and to start doing it without instruction
to start something
The exact meaning of starting something depends upon the context. For example:
"She did it on her own initiative." — started it without anyone telling her to
"She seized the initiative." — started actions that force others to react to her rather than her having to react to them
"the peace initiative" — a plan or the start of actions to bring about something
"a ballot initiative" — a proposed law that is started by citizen petition rather than by the legislature (applicable in many jurisdictions such as California)
A reluctant elevator-boy went for a box full of straw and some milk, to which he added on his own initiative a tin of large, hard dog-biscuits — one of which decomposed apathetically in the saucer of milk all afternoon.
This was his day off and with laudable initiative he had hurried out "to see."
There are no more uses of "initiative" in The Great Gatsby.