If you look at the kanji for aiki, you'll notice two distinct characters. The first (ai) one symbolizes union and harmony, and it's shaped like a house which is the ultimate symbol of union (family). The second character (ki) represents energy. It's made to look like a tea pot boiling, with the steam as a visual reminder of the energy being released. Together, the two characters mean harmony with energy or blended energy.
Aiki Jujutsu, the parent art to Aikido, uses this concept of blended energy to use the attacker's energy and aggression against them. By using non-resistance techniques and natural body movements, the attacker is easily redirected and subdued. This allows even the small and weak to defend themselves regardless of their attacker's size.