Time
<p><b >Time in physics</b> is defined by its <a href="http://en.wikipedia.org/wiki/Operational_definition" title="Operational definition" >measurement</a>: time is what a <a href="http://en.wikipedia.org/wiki/Clock" title="Clock" >clock</a> reads.<sup id="cite_ref-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Time_in_physics#cite_note-0" >[1]</a></sup> It is a <a href="http://en.wikipedia.org/wiki/Scalar(physics)" title="Scalar (physics)" >scalar</a> quantity and, like <a href="http://en.wikipedia.org/wiki/Length" title="Length" >length</a>, <a href="http://en.wikipedia.org/wiki/Mass" title="Mass" >mass</a>, and <a href="http://en.wikipedia.org/wiki/Electric_charge" title="Electric charge" >charge</a>, is usually described as a <a href="http://en.wikipedia.org/wiki/Fundamental_quantity" title="Fundamental quantity" class="mw-redirect" >fundamental quantity</a>. Time can be combined mathematically with other <a href="http://en.wikipedia.org/wiki/Physical_quantities" title="Physical quantities" class="mw-redirect" >physical quantities</a> to <a href="http://en.wikipedia.org/wiki/Formal_proof" title="Formal proof" >derive</a> other concepts such as <a href="http://en.wikipedia.org/wiki/Motion(physics)" title="Motion (physics)" >motion</a>,<a href="http://en.wikipedia.org/wiki/Kinetic_energy" title="Kinetic energy" >kinetic energy</a> and time-dependent <a href="http://en.wikipedia.org/wiki/Field_(physics)" title="Field (physics)" >fields</a>. <i ><a href="http://en.wikipedia.org/wiki/Category:Timekeeping" title="Category:Timekeeping" >Timekeeping</a></i> is a complex of technological and scientific issues, and part of the foundation of <i ><a href="http://en.wikipedia.org/wiki/Recordkeeping" title="Recordkeeping" class="mw-redirect" >recordkeeping</a></i>.</p>