When using the Poisson distribution, which parameter of the distribution is used in probability computations? What is

Question:

When using the Poisson distribution, which parameter of the distribution is used in probability computations? What is the symbol used for this parameter?

Distribution
The word "distribution" has several meanings in the financial world, most of them pertaining to the payment of assets from a fund, account, or individual security to an investor or beneficiary. Retirement account distributions are among the most...