Oxygen/ˈɑːksɪdʒən/ Definition 1 of 3Pro Tip 1/3Vital for Life Oxygen is often used to express the critical need for something. Water, like oxygen, is vital for survival. Pro Tip 2/3Emergency Situations Oxygen equipment is commonly referenced in safety and medical emergencies. The paramedics put an oxygen mask on the patient. Pro Tip 3/3Environmental Concern Discussions on oxygen may indicate environmental issues like deforestation. Trees are crucial because they produce oxygen.