The art and business of wizardry, considered as a whole: also, the act of going out to take care of some piece of wizardly business.
The English-language form of the word derives directly from the word "errand", which in its original usage routinely meant not only the task someone has been sent to do, but the journey one embarks upon to complete it. These early usages are the source of word formations like "knight-errant". As used by wizards, the word implies that the mission or errand in hand is one in which the wizard is acting on behalf of the Powers that Be.
A more specific usage, stating that someone is "on active assignment (of errantry)", means that the wizard has received a direct commission to fulfill a specific task. Normally these commissions come from the Powers via a given wizard's version of the Wizard's Manual, though they can come by more indirect means, such as a request from another wizard. Commissions can be declined, but normally a wizard is expected to have a compelling personal reason for refusing one. A wizard pursuing a piece of wizardly business not specifically commissioned by the Powers is nonetheless entitled to describe it to others as errantry. The Speech does, however, contain any number of words intended specifically for modifying the basic concept so that a traveling or working wizard can be precise about making it plain to colleagues encountered along the way that a given project is elective, and can be set aside to assist with a formal commission if necessary.
The declaration by the wizard that he, she or it is formally out on errantry normally means that local authorities (in worlds where wizardry is openly recognized) will immediately render the wizard whatever assistance he / she / it may require. In the Speech, there are any number of words by which the wizard can express the urgency of the quest or task they are engaged upon. However, for times when speed or confidentiality are an issue, there is hardly an astahfrith culture anywhere among the inhabited worlds that does not recognize and honor the commonest form of the formal avedictory, "I am on errantry, and I greet you!"
See also: High Road, the.