Different definitions of unit
In the days of Ken Uston, a "unit" was just the minimum bet at the tables where you played, or the minimum bet you would consider making, consistent with your overall cover strategy. This is what revereman apparantly means when he says "unit."
Today we have Optimal Betting Theory (OBT), and the way it works is you input a maximum to minimum bet ratio such as 4 to 1, or 12 to 1, or whatever else you want, and then OBT tells you exactly what all your bets must be for a given Risk of Ruin to a given Bankroll.
For example, OBT could tell you to bet $12.37, at TC less than or equal to +1, $17.97 at +2, $23.35 at +3, $39.44 at +4 and $49.48 at +5 and above. $12.37 would then be called your "unit" because it corresponds with the 1 in your 4 to 1 bet ratio. (Note $12.37 x 4 = $49.48) This is just an example, and the numbers will be wildly different, depending on the game, bankroll, desired RoR, etc.
But this is what math heads typically mean when they say "unit" nowadays, because "Hey! It's a cool theory!" -- even when it has no direct bearing on bets actually played (which tend to be multiples of $5, for obvious reasons). Neither type of unit relates very well to RoR or standard deviation without a lot more information.