Is sex always intercourse? Or is it oral sex? Would erotic phone conversations or kissing count as sex?

Much of the four hours of Mr Clinton's video-taped testimony addresses what was once considered a fairly clear-cut definition.

No more. Throughout the testimony, President Clinton firmly insists that in his view he did not have sex with Monica Lewinksy while prosecutors subtly insist that he did.

When Jane met Harry

In the testimony, Mr Clinton firmly stays by his definition of sexual relations: a person engages in sexual relations when the person knowingly engages in or causes - contact with the genitalia, anus, groin, breast, inner thigh, or buttocks of any person with an intent to arouse or gratify the sexual desire of any person.

But he also maintained that most Americans would define a sexual relationship as "sleeping together ... having intercourse".

He said: "If you said 'Jane and Harry had a sexual relationship' - and we are not talking about people being drawn into a lawsuit and being given definitions and great efforts being made to trip them in some way, but you are just talking about people in ordinary conversation - I bet that the Grand Jurors, if they were talking about two people they knew and said they had a sexual relationship, they meant they were sleeping together, they meant they were having intercourse together."

For Mr Clinton this assumption plus the Jones definition meant he did not have sex with Monica Lewinsky - just inappropriate intimate contact.

Keeping cool on hot issues

During his testimony, Mr Clinton was clearly irritated by prosecutors' questioning about whether he understood the meaning of sex as defined by the court. Even when pressed about Ms Lewinsky's testimony, he stuck to his story.

"I believe that at the time that [Monica Lewinsky] filled out this affidavit, if she believed that the definition of 'sexual relationship' was two people having intercourse, then she was accurate," he said.

"I believe that that is the definition that most ordinary Americans would give."

Asked if he defined sex as intercourse only, he replied: "No, not necessarily intercourse only, but it would include intercourse.

"I believe that the common understanding of the term, if you say, two people are having a sexual relationship, most people believe that includes intercourse."

"I would have thought that that's what nearly everybody thought it meant."

Truth

But the argument goes further than physical contact. The president and the prosecutor are wrangling over the definition of another word - truth.