定义：
激光装置产生的光。

激光是激光器产生的光。激光具有一些很特殊的性质，将它去其它光区分开： 

1. 激光通常以激光光束的形式传播，即它在一个特定的方向上传播且光束发散角适中。激光光束具有很高的空间相干性。这表示电场在不同位置处的相位关系是确定的。这也是激光光束传输很长距离后，横向并没有扩展很多的原因，同时也解释了它为什么可以聚焦为一个很小的点（激光光束的强聚焦性能）。 
2. 在很多情况下，激光光束也具有很高的时间相干性，等价于相干长度很长。这表示在相对长时间间隔内，或者说在很长的传播距离内，或者振荡周期数很大时，光束可以保持固定的相位关系。 
3. 大的时间相干性可由长的相干时间或相干长度来表征，与光束的窄光谱带宽（或线宽）有关。（这里不考虑复杂的超短脉冲列的情况，脉冲列的光学带宽很大，也具有很高的相干性，可参阅词条相干得到更多细节。）对于可见光的激光光束，代表该光束具有某一特定的纯色，例如，红色，绿色或者蓝色，而不是白色或者洋红。有些激光器可以在一定程度上进行波长调谐（例如，染料激光器）。长的相干时间容易形成激光斑纹，在激光光束射向金属表面时可以观察到。 
4. 大多数情况下，激光都是线偏振的。即电场在某一特定空间方向振荡（参阅激光辐射偏振）。 

激光还具有其它显著的特性： 

1. 激光有些是可见的，但是大多数激光处于其它非可见光光谱区域，尤其是近红外区域，这时人眼是察觉不到的。 
2. 激光并不总是连续的，但是会以短脉冲或者超短脉冲的形式传输。因此，峰值功率非常高（有些被放大的激光器系统功率可以大于1 TW (1012 W)），因此当脉冲时间源小于脉冲之间的时间间隔时，脉冲能量可以很大。 
3. 激光器的噪声特性也很丰富（参阅激光器噪声）。例如，激光器的振荡频率能够稳定到很窄的光谱范围。 

白炽灯发的光部分是可见光，部分是红外线，光谱带宽很高，不能被紧聚焦（由于空间相干性很低），因此不能以短脉冲的形式产生。 
激光会产生显著伤害，尤其是对人眼，还会对皮肤造成伤害，以及火灾等。激光脉冲通常很危险，因为激光的光强非常高，激光光束的不可见也增加了风险。参阅词条激光安全得到更多细节。

Definition: light generated with a laser device

Alternative term: laser radiation

More general terms: light

Laser light (laser radiation) is simply light generated with a laser device. Such light has some very special properties, which very much distinguish it from light with other origins:

• Laser light is usually delivered in the form of a laser beam, i.e. it propagates dominantly in a well-defined direction with moderate beam divergence. Such a laser beam has a high (sometimes extremely high) degree of spatial coherence. This means that the electric fields at different locations across a beam profile oscillate with a rigid phase relationship. Exactly this coherence is the reason why a laser beam can propagate over long distances without spreading very much in the transverse directions, and why it can be focused to very small spots (high focusability of laser beams).
• In many but not all cases, laser light also has a high degree of temporal coherence, which is equivalent to a long coherence length. This means that a rigid phase relationship is also maintained over relatively long time intervals, corresponding to large propagation distances (often many kilometers) or to huge numbers of oscillation cycles.
• The large temporal coherence, quantified with a large coherence time or coherence length, is associated with a narrow spectral bandwidth (or linewidth). (We exclude here the sophisticated case of trains of ultrashort pulses, which can have a large optical bandwidth but nevertheless a high degree of coherence; see the article on coherence for details.) For a visible laser beam, this means that it has a certain pure color, e.g. red, green or blue, but not white or magenta. Some lasers allow a degree of wavelength tuning (e.g. dye lasers). The large coherence length introduces a tendency for the phenomenon of laser speckle, i.e. a characteristic granular pattern which can be observed e.g. when the laser beam hits a metallic surface.
• In most cases, laser light is linearly polarized. This means that the electric field oscillates in a particular spatial direction (→ polarization of light).

Depending on the case, laser light can have other remarkable properties:

• Laser light may be visible, but most lasers actually emit in other spectral regions, in particular in the near-infrared region, which human eyes cannot perceive.
• Laser light is not always continuous, but may be delivered in the form of short or ultrashort pulses. As a consequence, the peak power can be extremely high – for some amplified laser systems well above 1 TW (10¹² W) – if the pulse duration is far below the temporal distance of the pulses, so that a large pulse energy is possible.
• The noise properties of lasers can also be very interesting (→ laser noise). For example, the oscillation frequency of a laser can be stabilized to stay within an extremely narrow range.

For comparison, light from an incandescent lamp is partly visible and mostly infrared, has a very high spectral bandwidth, cannot be strongly focused (because of low spatial coherence), and cannot be generated in the form of short pulses.

Laser light can cause significant hazards, particularly for the eye, but also for the skin, apart from fire hazards and others. Lasers pulses are often particularly dangerous, because they can have enormously high optical intensities, and invisible laser beams add specific risks. See the article on laser safety for details.