Coordinate systemIn geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is significant, and they are sometimes identified by their position in an ordered tuple and sometimes by a letter, as in "the x-coordinate". The coordinates are taken to be real numbers in elementary mathematics, but may be complex numbers or elements of a more abstract system such as a commutative ring.
Gravitational accelerationIn physics, gravitational acceleration is the acceleration of an object in free fall within a vacuum (and thus without experiencing drag). This is the steady gain in speed caused exclusively by the force of gravitational attraction. All bodies accelerate in vacuum at the same rate, regardless of the masses or compositions of the bodies; the measurement and analysis of these rates is known as gravimetry. At a fixed point on the surface, the magnitude of Earth's gravity results from combined effect of gravitation and the centrifugal force from Earth's rotation.
ChristmasChristmas is an annual festival commemorating the birth of Jesus Christ, observed primarily on December 25 as a religious and cultural celebration among billions of people around the world. A feast central to the Christian liturgical year, it is preceded by the season of Advent or the Nativity Fast and initiates the season of Christmastide, which historically in the West lasts twelve days and culminates on Twelfth Night.
Christmas carolA Christmas carol is a carol (a song or hymn) on the theme of Christmas, traditionally sung at Christmas itself or during the surrounding Christmas holiday season. The term noel has sometimes been used, especially for carols of French origin. Christmas carols may be regarded as a subset of the broader category of Christmas music. The first known Christmas hymns may be traced to 4th-century Rome. Latin hymns such as Veni redemptor gentium, written by Ambrose, Archbishop of Milan, were austere statements of the theological doctrine of the Incarnation in opposition to Arianism.
Line segmentIn geometry, a line segment is a part of a straight line that is bounded by two distinct end points, and contains every point on the line that is between its endpoints. The length of a line segment is given by the Euclidean distance between its endpoints. A closed line segment includes both endpoints, while an open line segment excludes both endpoints; a half-open line segment includes exactly one of the endpoints. In geometry, a line segment is often denoted using a line above the symbols for the two endpoints (such as ).
Line (geometry)In geometry, a line is an infinitely long object with no width, depth, or curvature. Thus, lines are one-dimensional objects, though they may exist embedded in two, three, or higher dimensional spaces. The word line may also refer to a line segment in everyday life that has two points to denote its ends (endpoints). A line can be referred to by two points that lie on it (e.g. ) or by a single letter (e.g. ).
Christmas cardA Christmas card is a greeting card sent as part of the traditional celebration of Christmas in order to convey between people a range of sentiments related to Christmastide and the holiday season. Christmas cards are usually exchanged during the weeks preceding Christmas Day by many people (including some non-Christians) in Western society and in Asia. The traditional greeting reads "wishing you a Merry Christmas and a Happy New Year".
Father ChristmasFather Christmas is the traditional English name for the personification of Christmas. Although now known as a Christmas gift-bringer, and typically considered to be synonymous with Santa Claus, he was originally part of a much older and unrelated English folkloric tradition. The recognisably modern figure of the English Father Christmas developed in the late Victorian period, but Christmas had been personified for centuries before then.
Hyperbolic partial differential equationIn mathematics, a hyperbolic partial differential equation of order is a partial differential equation (PDE) that, roughly speaking, has a well-posed initial value problem for the first derivatives. More precisely, the Cauchy problem can be locally solved for arbitrary initial data along any non-characteristic hypersurface. Many of the equations of mechanics are hyperbolic, and so the study of hyperbolic equations is of substantial contemporary interest. The model hyperbolic equation is the wave equation.
Christmas giftA Christmas gift or Christmas present is a gift given in celebration of Christmas. Christmas gifts are often exchanged on Christmas Eve (December 24), Christmas Day itself (December 25) or on the last day of the twelve-day Christmas season, Twelfth Night (January 5). The practice of giving gifts during Christmastide, according to Christian tradition, is symbolic of the presentation of the gifts by the Three Wise Men to the infant Jesus. Gift-giving in general is an ancient tradition that came to be associated with the Christian feast of Christmas.
Christmas EveChristmas Eve is the day before Christmas Day, the festival commemorating the birth of Jesus. Christmas Day is observed around the world, and Christmas Eve is widely observed as a full or partial holiday in anticipation of Christmas Day. Together, both days are considered one of the most culturally significant celebrations in Christendom and Western society.
Christmas and holiday seasonThe Christmas season or the festive season (also known in some countries as the holiday season or the holidays) is an annually recurring period recognized in many Western and other countries that is generally considered to run from late November to early January. It is defined as incorporating at least Christmas Day, New Year's Day, and sometimes various other holidays and festivals. It also is associated with a period of shopping which comprises a peak season for the retail sector (the "Christmas (or holiday) shopping season") and a period of sales at the end of the season (the "January sales").
