The substance common to all conscious entities which defines an individual's place in space-time and dictates the relationship between that individual and the rest of history. Describing biodata is difficult, as a full understanding seems to require an inherent grasp of time-active modeling only native to the Time Lords-- perhaps the best simple definition is "Time DNA".
In the same way that DNA shapes an individual's genetic form, biodata shapes an individual's course through the continuum, and can be regarded as a strand of information running through all four mundane dimensions: the shape of the biodata strand, as the individual moves along his or her own timeline, is the true shape of that individual's life.
More than that, biodata is also a reflection of an individual's path through existence. Consider: according to the standard model of quantum thinking, nothing provably exists in the universe-- the universe is just a series of probabilities of what may or may not happen, what may or may not be measurable, and no scientific theory can prove that any one of these possibilities is definitively true. Only the presence of a conscious observer can collapse the possbilities, until only at least one state is at least seen to exist. This means that, if things only become actual once an observer is present to give them meaning, then these strands of meaning make up the real mass of the universe, as opposed to physical matter-- and this furhter implies that as an individual moves through the four "common" dimensions-- just living their life--they leave a trail through reality--a path connecting all the strands of meaning they have made actual. Thus, with careful reading these strands could be read to make out the true shape of the indivisdual's life, which would bear no relation ot their flesh and blood form at all.
Biodata evolves; the Biodata of higher species (conceptuals, gods, Time Lords etc) able to influence the biodata of lesser species.
It has been theorized that an individual is their biodata strand, and their their body at any point during their existence is merely akin to a cursor pointing out an area on that strand. Thus the true individual would be an enormous mass of collpased probabilities and transtemporal information that is invisible and innconcievable yet very real. In this model of reality every conscious being is the size of whole worlds with no solid dividing line between one life and the next.
If one knows how to read an individual's biodata strand, one can read that individual's entire history from every facet of his or her past to every moment of his or her future. The Time Lords, Faction Paradox, the Celestis and the Sisterhood of Karn do know how to read biodata, but the procedure is difficult (and for the first three groups anyway) requires extensive use of technology.
It is far easier to read past biodata than future biodata, making future prediction based on it an inexact science erroring out to greater degrees the further ahead one looks.
A Time Lord's biodata changes with each regeneration, the new regeneration and its potential being added on to all prior points of the Time Lord's biodata each time.
Biodata, being temporal in nature, adheres to only a specific instantiation of a being, it does NOT carry over to clones. Clones will generate their own, unique biodata. (To TRULY copy someone their entire relationship to history must also be copied-- the easiest way is to diffract the subject's timeline and find / extract near parallels)