Bhagavad Gita Verse 26, Chapter 18

muktasangonahamvaadee dhrityutsaahasamanvitaha |
siddhyaasiddhyornirvikaaraha kartaa saattvika ucchyate || 26 ||
 
One who is free from attachment, who does not support egoism, filled with fortitude and enthusiasm, unperturbed in success and failure, such a doer is called saattvic.
 
muktasangaha : free from attachment
anahamvaadee : one who does not support egoism
dhrityutsaahasamanvitaha : filled with fortitude and enthusiasm
siddhyaasiddhyoho : in success and failure
nirvikaaraha : unperturbed
kartaa : doer
saattvika : saattvic
ucchyate : is called
 
Let us recap what we have seen so far. Jnyaana or knowledge shapes our world view and prompts our actions. It is a product of Prakriti and is of three types. Karma or action itself is also of three types. Now, Shri Krishna starts a new topic and describes three types of the kartaa, the doer. The doer is nothing but the state of our mind from the start of an action to its end. In other words, the same action can be performed differently by different types of doer. We see this in our lives as well. No two people will perform a task in quite the same way.
 
This shloka describes a doer who is of the nature of sattva, of is sattvic. Shri Krishna says that a saattvic doer is prepared for any outcome of the action, be it success or failure. They will not let external circumstances change their state of mind. If the action fails, they do not dwell on it but learn from their mistakes and move on. Many top athletes have this quality, since they have to continually compete throughout their career, and cannot afford to get depressed or elated with each outcome.
 
In addition to success or failure, we also encounter temporary setbacks during the action. A sattvic doer possess extreme endurance to deal with these setbacks. Their enthusiasm to do their duty lifts them out of any temporary sense of sorrow. In the second chapter, Shri Krishna had emphasized the importance of fortitude, also known as titikshaa. Their utsaaha, their enthusiasm makes them stronger since they are not perturbed by outcome. They are also not concerned about egoism, about tooting their horn in front of others, since they are unconcerned about how good or bad they will look in front of others.
 
Freedom from attachment of any kind is the crucial quality of the saattvic doer. They are not attached to the outcome of the action, to the action itself, or to their sense of individuality, their ego. They have achieved a high level of vairagyaa, of dispassion from the pulls of the material world.