tetrahedral intermediate
Imagine a flat tabletop being pushed up into a pyramid. That is what happens to a carbonyl carbon the instant a nucleophile attacks it: the carbon goes from flat (sp2, three bonds in a plane) to a four-cornered, pyramid-like geometry (sp3, four bonds spread out in space). The species formed at that moment is the tetrahedral intermediate, the crucial halfway point of nearly every carbonyl reaction.
Before attack, the carbonyl carbon has three groups around it at 120-degree angles and a double bond to oxygen. When the nucleophile arrives, it forms a new bond to the carbon, the C=O pi bond breaks, and that electron pair moves onto the oxygen, turning it into a negatively charged alkoxide (or, if protonated, a neutral OH). The carbon now has four single bonds pointing to the four corners of a tetrahedron. This intermediate is usually short-lived and high in energy.
What happens next defines the reaction's whole personality. In aldehydes and ketones, the tetrahedral intermediate has no good leaving group, so it simply grabs a proton and stays put: that gives addition (a new alcohol or related product). In carboxylic acid derivatives, by contrast, the intermediate can expel a leaving group and re-form the C=O, giving substitution instead. So this one geometric switch is the fork in the road between addition and substitution.
When hydride from NaBH4 attacks acetone, the fleeting tetrahedral intermediate is the alkoxide (CH3)2CH-O- with the carbon now sp3; protonation by water then gives isopropanol.
The carbon flips from flat sp2 to pyramidal sp3 at the moment of attack.
It is called an intermediate, not a transition state: it is a real (if short-lived) species sitting in a dip on the energy diagram, whereas a transition state is the fleeting peak between species.