Bhagavan is the one who has bhaga,

 Bhagavan is the one who has bhaga, the six-fold virtues in absolute measure.

भगः अस्य अस्ति इति भगवान्
bhagaḥ asya astiiti bhagavān

The one who has bhaga is called bhagavan. It is said in the Vishnu Purana (6.5.74)

ऐश्वर्यस्य समग्रस्य वीर्यस्य यशसः श्रियः।
ज्ञान-वैराग्ययोश्चैव षण्णां भग इतीरणा॥
aiśvaryasya samagrasya vīryasya yaśasaḥ śriyaḥ |
jñāna-vairāgyayoścaiva ṣaṇṇāṁ bhaga itīraṇā ||

1 Total and absolute overlordship,

2 power,..virya;

3 wealth, sri;  

4 dispassion, vairagya

5 fame ,yasas;

6 knowledge, jnana

These six absolute virtues, then, constitute bhaga and the one who has this six-fold bhaga is Bhagavan.

To have all knowledge, jnana, is to be free from all ignorance. Thus the one who has all jnana does not require a mind, perception, or a means of knowledge with which to know. If we require a mind to know, there is always ignorance. Thus, whoever is endowed with a mind in order to know cannot be Bhagavan. Bhaga, therefore, is jnana, all knowledge.

Total, absolute, dispassion is also called bhaga. The one who has total, absolute dispassion, vairagya, has no longing, no insecurity. He or she is full.

Virya is absolute power or sakti meaning the capacity to create, to sustain, and to resolve. The one who has this absolute power is called the Almighty.

Absolute fame, yasas, means all fame, including your own or anyone else’s. This is also bhaga. One who is endowed with the ability to sing, for example, may gain some fame, a ray of glory, but that fame belongs to the Lord, the one who has all fame.

The Lord also has all wealth, all resources - Sri. Any wealth you may have really belongs to Bhagavan, the Lord, and you are only a trustee of whatever resources you happen to have

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Finally, the one who is not caused, who is not ruled by anyone, who is not subject to the laws of someone else, has the bhaga called aisvarya - overlordship.

We, as individuals have to go by the laws of nature; we cannot go against them. Even an engineer who is responsible for planning, commissioning, and running a thermal plant cannot touch a live wire without facing the consequences. In other words, the engineer cannot behave as he or she likes merely because he or she caused the electricity to be generated. It was because of the laws alone that the engineer was able to generate electricity in the first place

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The one who does not subject himself to the law or laws of another is the Lord. If the Lord is ruled by someone else, then that someone else becomes the Lord; and it is this Lord that we are talking about.

These six absolute virtues, then, constitute bhaga and the one who has this six-fold bhaga is Bhagavan.