This depends on what field of study you are talking about.
The scientific philosopher Karl Popper has defined the term "conjecture" as a hypothesis that seems to be true but which has not yet been proven. He says that the creation of conjectures is the basis of science. A person comes up with a theory or a conjecture and then tries to falsify or prove it.
In this case, then, any scientific theory that is falsifiable but has not been proven true or false is a conjecture. For example, the argument that a rise in global temperature will create more and stronger hurricanes is a conjecture because it can be falsified but has not yet been proven one way or the other.
Examples of conjectures:
1) The lengths of the opposite sides of a parallelogram are equal.
2) the adjacent angles of a parallelogram are supplementary:
3) The sum of all angles of a triangle is 180 degrees.
4) The adjacent base angles of an isosceles triangle are congruent.