Alpha
The first sat of a whole bitcoin: a sat whose number is divisible by 100,000,000. On its own it is not a category; combined with others it is.
How a sat qualifies
Ordinal theory numbers sats from 0, so the sat numbers fall into runs of 100,000,000, one per bitcoin: sats 0 to 99,999,999, then 100,000,000 to 199,999,999, and so on. Sat s is an alpha when s is divisible by 100,000,000, the first sat of such a run. Its counterpart at the other end is the omega.
These are bitcoin in the numbering, not coins in anyone's wallet: an output worth one bitcoin need not hold one whole run.
Exactly 21,000,000 alphas will exist, from sat 0 to sat 2,099,999,900,000,000. The last run of the numbering is never completed (only 97,690,000 of its sats will be mined), but its first sat is still an alpha.
Alphas and blocks
While the subsidy was 50 or 25 bitcoin, every block began on a whole bitcoin, so every block from 0 to 419,999 starts with an alpha: an alpha uncommon (or a rarer tier). After that only some blocks do: every second block from 420,000, then fewer at each halving, and from block 2,100,000 on none. Most alphas fall mid-block: a block of the first halving epoch, at 50 bitcoin, holds 50 of them.
An alpha at a round number of bitcoin also carries a milestone type: kilosat, decasat, centasat or megasat.
On this site
There is no plain Alpha category: an alpha with no other type is not tracked. Alphas are tracked where they combine with something else: the first sat of a block (Alpha uncommon), a milestone, or a historic place such as vintage, Nakamoto blocks, pizza or Silk Road. Those categories are listed below.