A clerk (/klɑːrk/ or /klɜːrk/) is a white-collar worker who conducts general office tasks, or a worker who performs similar sales-related tasks in a retail environment (a retail clerk). The responsibilities of clerical workers commonly include record keeping, filing, staffing service counters, screening solicitors, and other administrative tasks.
The word clerk is derived from the Latin clericus meaning "cleric" or "clergyman", which is the latinisation of the Greek κληρικός (klērikos), "of the clergy".
The association derived from early medieval courts, where writing was mainly entrusted to clergy because most laymen couldn't read. In a medieval context, the word clerk meant "scholar". Even today, the term Clerk regular designates a type of regular cleric. The cognate terms in some languages, e.g., Klerk in Dutch, became restricted to a specific, fairly low rank in the administrative hierarchy.
In quantum mechanics, the position operator is the operator that corresponds to the position observable of a particle. The eigenvalue of the operator is the position vector of the particle.
In one dimension, the square modulus of the wave function, , represents the probability density of finding the particle at position
. Hence the expected value of a measurement of the position of the particle is
Accordingly, the quantum mechanical operator corresponding to position is , where
The circumflex over the x on the left side indicates an operator, so that this equation may be read The result of the operator x acting on any function ψ(x) equals x multiplied by ψ(x). Or more simply, the operator x multiplies any function ψ(x) by x.
The eigenfunctions of the position operator, represented in position space, are Dirac delta functions.
To show this, suppose that is an eigenstate of the position operator with eigenvalue
. We write the eigenvalue equation in position coordinates,
In financial trading, a position is a binding commitment to buy or sell a given amount of financial instruments, such as securities, currencies or commodities, for a given price.
The term "position" is also used in the context of finance for the amount of securities or commodities held by a person, firm, or institution, and for the ownership status of a person's or institution's investments.
In derivatives trading or for financial instruments, the concept of a position is used extensively. There are two basic types of position: a long and a short.
Traded options will be used in the following explanations. The same principle applies for futures and other securities. For simplicity, only one contract is being traded in these examples.
Position in team sports refers to the joint arrangement of a team on its field of play during a game and to the standardized place of any individual player in that arrangement. Much instruction, strategy, and reporting is organized by a set of individual player positions that is standard for the sport.
Some player positions may be official, others unofficial. For example, baseball rules govern the pitcher by that name, but not the shortstop, where pitcher and shortstop are two of baseball's nine fielding positions.
For information about team or player positions in some particular sports, see: