Nirvana is the soteriological (i.e., having to do with salvation) release from samsara (life as it is usually lived, imprisoned by karma, delusion, and dukkha). In some versions of Buddhism, nirvana is the ultimate goal of liberation — Shakyamuni’s death is often referred to as his entrance into nirvana. In the Mahayana, nirvana is entrance into full liberation, which allows beings to liberate others. Nirvana is often confused with samadhi— the latter is a temporary state, which we move in and out of, and which only a fool would want to stay in permanently. You are free to say that you have attained samadhi, but by definition if you have a human, or any other kind of body, you have not attained nirvana.
For more information see https://en.wikipedia.org/wiki/Nirvana_(Buddhism)