Senshin means purified spirit & enlightened mind / attitude. It is commonly seen as the Buddha mind and what the perception of the founder of Aikido, Morihei Ueshiba was studying and using the function of Aikido to pass on this message.
To fully embrace Senshin is truly rare, but achievable, what makes it hard is that it can't be forced and takes a lifetime of practice in controlling your thoughts and purifying your spirit. You must at all times be sincere in your Martial Art training, your every action, your spirit, mind and body in every moment on and off the mat. It is essentially equivalent to becoming enlightened and will exceed the scope of daily Aikido.
There are certain things in life that we expect to happen or achieve. When these things do not reach our expectations, they turn into disappointments and dissatisfactions. In order for you to achieve a purified spirit, you have to wash away excessive expectations, and live in the moment facing reality and accept the future with a normal heart. Then the concept of Senshin can start to be understood.
This is the highest level of Budo in your Aikido and Martial Art training, at this level, you will hold all life as sacred, you will be able to perceive how everything fits together to make the whole. You will understand how each part of the Universe is connected and how something that affects one part of the Universe will ultimately affect us all. You begin to see the world in a totally different way which affects your thoughts and intentions making them pure and sincere and your mind and spirit will become one.
“The Art of Peace is medicine for a sick world. There is evil and disorder in the world because people have forgotten that all things emanate from one source. Return to that source and leave behind all self centered thoughts, petty desires, and anger. Those who are possessed by nothing possess everything."