The most difficult mathematics is that which you do not know. A surprising amount of mathematics is actually easy once you've learned it. Of course, once you learn the easy stuff, then you have to start tacking the deep stuff, and that gets harder.
An isosceles triangle has two sides of equal length. An isosceles triangle also has two angles of the same measure; namely, the angles opposite to the two sides of the same length; this fact is the content of the isosceles triangle theorem, which was known by Euclid.
See more about definitions at Precision of Definitions. Example of a definition: An isosceles triangle is a triangle with two congruent sides. (if-then form) If a triangle is isosceles, then the triangle has two congruent sides. (reversed if-then form) If a triangle has two congruent sides, then the triangle is isosceles.