A conjunction is an aspect centered on 0°, meaning two chart points occupy the same or nearby zodiac longitude. They need not have identical latitude or appear visually merged in the sky.
An orb policy determines how far from 0° the aspect may be and still receive the name. Astrology usually interprets a conjunction as a concentrated combination of the two symbolic functions, but the quality of that combination depends on context. “Combined” does not automatically mean easy or difficult.
The exact separation is calculated geometry; the allowed orb and symbolic meaning are conventions. Compare with an opposition, which is centered on 180°.