I don't define it. In fact I never used the term or heard of it until the MMA guys started telling me and all the other guys doing traditional arts that we don't do that. Funny, we have been doing that. We just never called it that. Instead we simply said either you train realistically or you don't. We TMA guys also never described fighting as being Stand-up, ground game, a grappling art, a striking art,.... And why would we? We just thought that all that was part of the art we studied. At least it used to be before people started telling me that we don't do that.

....

Aliveness training (to me) is an "open" response defense training activity where an individual is given an attacker ( or more than one) and is told to defend themselves against the coming assault. There is no predetermined attack scenario, and there are no predetermined defense techniques either. The assailants can use almost any attack that is non-critical, and the defender can do the same. The contest is over when the defender is overcome, or the attacker(s) is/are physically vanquished.

Because of the intensity, and the potential for injury, ALL pertinent safety gear should be required, and students who wish to partake in "aliveness" training should be of a sufficient skill to avoid injuring themselves or others. When first introducing lower level students to this training they should be "confronted" by instructors, or high ranking students, that have the training and ability to control the situation to make sure the students are not injured while evaluating their skill level during the training event.

"aliveness" is the way we trained before anyone even understood "aliveness", which I feel is an ill thought out term.

You kids are just discovering this now.

It's not well known because most early practitioners were given "partial arts" instead of being trained in martial arts.

We call it "hard sparring". Pretty much what pugpaws2 said.

You have sparring - Light/Intermediate Match, trying to win yet not hurt each other, trading blows, etc.

Than you have "realistic" sparring or what we "hard" sparring. You attack your uke like you want to truly hurt them. You don't of course, but it's a rushed more brutal form of attack. As mentioned "There is no predetermined attack scenario, and there are no predetermined defense techniques either. The assailants can use almost any attack that is non-critical, and the defender can do the same".

It's hard at first but effective.

As a catch phrase used to deride traditional martial arts by people who fail to understand the need for static training methods to develop the technique required for self defence..

Training for practicality I guess? You use techniques and refine your tactical wheels so that you can be spontaneous to knew and old experiences in combat?

As a marketing tool, apparently, devised in a MMA gym.

Free sparring just reeks of traditional martial arts... and we couldn't have that!

Bogeyman's answer is perfect