Michelle and The Doctor (Ten)
"A Time Lord is... a species of alien," he said, after a moment's consideration. "It is what those from Gallifrey are called." He considered for another moment before considering. " Time Lords are so named for their command of time travel technology and their non-linear perception of time. ...wibbly wobbly timey wimey stuff."
He hoped he was explaining it well enough. Sometimes he forgot things. Sometimes he purposely left things out. "I am the last of the Time Lords. So is she."
Then, at Michelle's question of there being two Doctors, "That's where it gets a bit complicated." As though the rest of the information he had given her weren't complicated. "I tend to... die. And regenerate after dying. How I appear now is not how I appeared before the last time I died -- I look and sometimes act completely different."
Almost realizing that it didn't explain the existence of another Doctor in Madison Valley, he continued. "SHe is a regeneration of me. ...I think there were two other regenerations of me before she came to be. As I said, wibbly wobbly timey wimey stuff."