What is dilution?
Dilution is what happens to your ownership percentage when a company issues new shares. You keep every share you already hold. There are simply more shares in total, so the same holding is a smaller percentage of a bigger company.
What it looks like
Two founders hold 6,000,000 shares between them, alongside a 500,000 share option pool. That is 6,500,000 fully diluted, so each founder owns about 46.2%.
They raise $1.5M at a $5.2M pre-money valuation, which prices shares at $0.80. The round issues 1,875,000 new shares and the total becomes 8,375,000.
Each founder still holds 3,000,000 shares. They now own 35.8%. The investors hold 22.4% and the pool is 6.0%.
Nothing was taken from anybody. Dilution is only a loss if the company is worth less after the raise than before it.
What causes it
A priced funding round
SAFEs or convertible notes converting
Creating or topping up an option pool
Options or warrants being exercised
Why two people quote different percentages
A percentage only means something once you say what it is measured against.
Outstanding counts shares actually issued today.
Fully diluted counts everything that could become a share: unexercised options, the unissued pool, unconverted SAFEs and notes.
The same holding produces two different percentages. Check which basis a number is on before you compare it to anything.