1. The problem asks: Two numbers that are the same distance from 0 on the number line but are on opposite sides of 0 are called __________. 2. The key concept here is about numbers on the number line relative to zero. 3. Numbers that are the same distance from zero but on opposite sides are called **opposites**. 4. For example, 3 and -3 are opposites because both are 3 units away from 0 but on different sides. 5. Therefore, the correct answer to fill in the blank is **opposite**.