|Definition - A tense that indicates the future commencement of an action.|
(1) I am going to eat.
(2) They are about to leave.
Oxford English Dictionary - The term's first citation is from 1824:
"‘I am going to write’; ‘I am about to write’. These have been called the Inceptive future, as they note the commencement of an action."
(L. Murray Eng. Gram. (ed. 5) I. 128,)