Elements of activity diagrams

Element	Syntax
Action	`action "Name"`
Initial and final nodes	`start`, `stop`, `end`
Decision and merge	`if`, `switch`, `while`, `repeat`
Fork and join	`fork { ... } and { ... }`
Object node	`object "Name"`
Signals	`send "Name"`, `accept "Name"`
Partition	`lane "Name" { ... }`
Note	`note: text`

diagram activity "All the activity elements"
start
action "Receive"
if "valid" {
  action "Process"
} else {
  action "Reject"
}
stop

See Activity diagrams for how to write one.