How did the Buddha define success?

Buddha never defined success because nobody is a success. Sooner or later what originates in dust goes to dust with his or her accomplishments. Every effort which originates in desire implies rebirth and pain of going through sufferings of another life. Billionaires and millionaires suffer just like ordinary folks. Buddha talked about getting out of misery not to be comfortably miserable which successful people are.

If your goal is to be inside a golden prison with all the comforts then Buddha is not for you. Infact you should stop even thinking about being spiritual. But if you become successful because you tried to do your best without desiring it then you are spiritual. That way you will not need Buddha or any other guru or religion because you are just following your nature. That path is also termed as Karma yoga in Hinduism. But very few succeed because success brings in ego of accomplishment. You can be spiritual by just being you. All real artists are spiritual by nature. I have talked to many and figured out that they do not need much help in spirituality. Self actualization I think is a better alternative to taking path of self realization as of now for you.

The Buddhist goal is Nirvana and on jet to reach it is called a Sekkha. Success comes then you have reached the goal and free from all stress is accomplished (Asekkha).

Even you are are not fully accomplished, the practice may help you manage your day to day stress, until one day you do not get stressed any more.

Also do not procrastinate doing the tasks as this is the last day or moment. This would help in your worldly aspirations also.

First thing, "Only those fears death, who have not lived wisely", not sure if Gautama Buddha said this. Anyway assuming that he did, what he means is, one day you are going to die, and at that moment you should not be surprised. This can also be rephrased as, Do everything with focus and involvement, as if it is the last thing you would do.

It is not about spirituality versus something else. It is a general life thing. Nowadays people are distracted with 200 things on their minds because they got too involved in thinking/creating ideas rather than doing/implementing. People are taking their life for granted so much that when it is over, they have lost the time to live life rather than think about life. That seems to be what you are busy with right now. Your mind is playing with you. And you are playing along unconsciously. If you were conscious, then you would know it and start living life rather than unnecessarily thinking.

Secondly, Gautama Buddha's goal is enlightenment and the end of suffering. This is single mindedly looking inward to find out what is the nature of suffering and how to end it. If he accomplishes that, that is success. What is your goal? You need to define that for yourself.

Ultimately, know that you will die one day and thus live life consciously. And of course thinking/mind is required for planning and executing an activity. It is not of much use otherwise.

The sutta that you should live by, if you want to achieve success in your lay life - The layman’s welfare - is Dighajanu (Vyagghapajja) Sutta: Conditions of Welfare. In this sutta Buddha instructs rich householders how to preserve and increase their prosperity and avoid loss of wealth. This is The Buddha's Teachings on Prosperity.

The four conditions conduce to a householder's wellbeing and happiness in this very life are the accomplishment of persistent effort (utthana-sampada), the accomplishment of watchfulness (arakkha-sampada), good friendship (kalyanamittata) and balanced livelihood (sama-jivikata).

Here, endeavour is that effort put forth in earning money and wealth for the advancement of your life. Whatever you do in earning your living, you have to be an expert in it. You have to be knowledgable of the field of business. You should not be lazy but be enthusiastic in doing your work. Deliberation and skillfulness are some of the necessary elements to attain success. When you organise businesses skillfully your economy will, no doubt, increase. It enables you and your family members to live at ease without difficulties, troubles and worries.

Then you should know the importance of protecting wealth earned through enthusiasm and righteously working hard. Having known how hard earned wealth is protected and how it gets destroyed, you should know to protect it so that the collected wealth is not lost.

Good friendship is to associate with good people, people who follow a spiritual path. Your life has to have a moral foundation to enjoy external achievements and success. Material success alone is not enough, there should be an association with good friends. Good friends will guide you in right and wrong. They recommend a spiritual path while practising it. If association with a particular person leads to our welfare then that person can be regarded as a Good Friend regardless of his or her age. If you find a person with many good qualities in his life, whether elder or young, either from your own village or suburb or township, it is then good to form an association with him or her. Next is a balanced life. It is the life that you live in accordance to your means and station in life. You must know how to spend money and live an unburdened, debtless life per the state of your income. Especially expenditure should be balanced to your revenue. You should also know how to lead a life with nothing exceedingly luxurious nor extremely austere but should strive to live a peaceful harmonious life following the middle way. You must be especially mindful not to have a competitive attitude regarding lifestyles.

Now, the reasons for the welfare and happiness in the life after death are to be considered. There are four factors that bring happiness to the life after death. The accomplishment of faith (saddha-sampada), the accomplishment of virtue (sila-sampada), the accomplishment of charity (caga-sampada) and the accomplishment of wisdom (pañña-sampada).