Transaction
Data & DatabasesAll of these changes, or none of them.
A group of changes applied as one unit. Without it, "take money from A" can succeed while "give money to B" fails. Any operation touching two things at once needs one.
See also Durability, Integrity, Concurrency
Transactional email
InfrastructureMail sent because a user just did something.
Password resets, receipts, confirmations: expected within seconds by a person who is waiting for them. Distinct from marketing mail, which goes out on your schedule and needs consent. Mixing the two costs you the reputation that gets the urgent one delivered.
See also Deliverability, Consent, Bounce