A trinomial is a polynomial with three terms.
FOr example:

The expression above has 3 terms, so it can be called a trinomial.
A perfect square trinomial is a trinomial that be factored into a binomial(a polynomial with 2 terms) and squared or multiply itself.

Given the trinomial:

Let's factor the trinomial:

Take the perfect squaure trinomial rule: a² + 2ab + b² = (a + b)²

Therefore, to identify a perfect square trinomial, the middle term must be two times the product of the numbers being squared in the first term and the last term.
ANSWER:
The middle term of the trinomial must be two times the product of the numbers being squared in the first term and the last term.