Zazen is Zen sitting meditation or sitting in controlled conditions to practice mindfulness.
Zazen has certain traditional guidelines that make meditation more effective, but in reality, other positions can be zazen if they work better for you, develop your self-discipline, and are practiced with full awareness.
Standard zazen posture is to sit on a cushion in the lotus position, Japanese sitting pose, half-lotus, or cross-legged, but if these are uncomfortable, you can practice zazen in a chair.
Traditionally in zazen, your hands form the cosmic mudra- one hand cupped in the other, your thumbs meeting to form an oval just below your navel.