Reflection symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or bilateral symmetry is symmetry with respect to reflection. That is, a figure which does not change upon undergoing a reflection has reflectional symmetry.
Winter (// WIN-tər) is the coldest season of the year in temperate climates, between autumn and spring. It is caused by the axis of the Earth in the respective hemisphere being oriented away from the Sun. Different cultures define different dates as the start of winter, and some use a definition based on weather, but when it is winter in the Northern Hemisphere it is summer in the Southern Hemisphere, and vice versa. In many regions, winter is associated with snow and freezing temperatures. At the winter solstice, the days are shortest and the nights are longest, with days lengthening as the season progresses after the solstice.