Bhagavad Gita Chapter 13, Verse 2

श्री भगवानुवाच | इदं शरीरं कौन्तेय क्षेत्रमित्यभिधीयते | एतद्यो वेत्ति तं प्राहुः क्षेत्रज्ञ इति तद्विदः ||

śrī bhagavān uvāca idaṁ śarīraṁ kaunteya kṣetram ity abhidhīyate etad yo vetti taṁ prāhuḥ kṣetrajña iti tadvidaḥ

The Blessed Lord said: This body, O son of Kunti, is called the field (kshetra). He who knows this field is called the knower of the field (kshetrajna) by those who know of both.

Krishna gives the simplest, most direct definition: your body is the field (kṣetra). You—the awareness that knows the body—are the knower (kṣetrajña). This isn't philosophy. It's fact. Your body is what you experience. You are what experiences it. Most people live their entire lives thinking they are the body. They say 'I am tired' when the body is tired. They say 'I am sick' when the body is sick. They say 'I am old' when the body ages. But you're not the body. You're the awareness that knows the body is tired, sick, or aging. This distinction seems simple, but it changes everything. When you recognize yourself as the knower, separate from the field, you stop suffering from every physical change. The body changes, but you remain constant.