The backing of an affirmation or forecast by offering to forfeit, in case of an adverse issue, a sum of money or some article to one who, by accepting, maintains the opposite with a corresponding stipulation.
Though a bet be and often is, null and void in the eyes of the law, yet it may be a valid contract binding in conscience if the object is honest and if it fulfills the following conditions:
(a) the parties must have the free disposal of what they stake and both must bind themselves to pay if they lose;
(b) both must understand the matter of the bet in the same sense;
(c) neither must have absolutely certain evidence of the truth of his contention which he does not reveal to the other.
New Catholic Dictionary