One of the two parts of the Bible, did you say? There must be more to it than that, or we would just call them Part One and Part Two. Anyway, other books aren't divided into testaments, and nor are our favourite television programmes.
All right. So the two parts of the Bible are about different things - an old order, based on Abraham and Moses and the Law, and a new order, ushered in by Jesus. (Or, if you like, about the same thing - God's relationship with us all - but in two different ways.) Perhaps that's what a testament is.
Then we remember that in German and in French, your testament is what you write before you die - your instructions about how to dispose of your property. So it is in Scotland, though in English law it is used almost solely in the phrase: "This is my last will and testament." And didn't Jesus die for us? So perhaps it has something to do with that?
Yes and no. (And that is why we keep using the jargon.) Behind our word lies the Latin word testamentum, and the Greek word diatheke. For the ancient Romans and the ancient Greeks, these words meant just that - "how you say you want to dispose of your goods before you die".
But, to complicate things, the team of 72 scholars who (it is said) were sent to Alexandria around 250 BC to translate the Bible (the Old Part, of course) into Greek, decided that diatheke was the best word to translate the Hebrew word berit. But berit does not mean how-you-want-to-dispose-of-your-goods-when-you-die. It means an agreement. And very often it means a one-sided agreement - a promise made by God.
So God made a one-sided agreement with Noah (Gen. 6:18), with Abraham (Gen. 17:2), with Moses (Ex. 34:38). English translations usually call this a covenant - another piece of (lawyers'!) jargon meaning (more or less) a one-sided agreement.
Jeremiah foresaw that this old way of things would be replaced by a new "agreement", a new testament, a "new covenant." God declares: "I will put my law within them, and I will write it upon their hearts, and I will be their God, and they will be my people." (Jer 31:31-33) And Jesus at the Last Supper shared wine with those who learnt from him, and said it was "the new covenant in his blood" (Lk 22:20, 1 Cor 11:25) - "a new covenant, not in a written code, but in the spirit," said Paul to the Corinthians. A new relationship.
And in another letter to the Corinthians (2 Cor 3:6-16), Paul contrasts the two kinds of relationship, and mentions "reading" the old testament, or the old covenant - even though the "New Testament" had not yet been carefully defined. (In fact, most it hadn't even been written down at the time!)
So a testament is a one-sided agreement: something offered for nothing. The writer of the Letter to the Hebrews tells us that a testament requires a death, or a sacrifice (Heb 9) - but such a testament does not mean leaving-your-goods-when-you-die. The New Testament is the tale of Christ's sacrifice for us - to confirm an agreement - no, not an agreement, but a promise, a promise made by God to us, of a new relationship.
So thanks for asking the question, Philip! (And thanks for reading the answer, the rest of you!) The New Testament is really the Writings about the New Promise. And in it we read what this promise is, and what it means to share it in our hearts. Good news, isn't it!
HD