What does 惊鸿 mean in this context?
显然这出自一篇网文（玄幻或武侠之类）。惊鸿即为受惊而翀的鸿雁，“翩若惊鸿”就是盛赞洛神之体态轻盈。言归正传，这里就是一类仙侠玄幻小说常用设定罢了——此剑既唤作 惊鸿剑，就真的能够化为惊鸿了。或是极言其变换移形之迅捷，好突出它的厉害。事实上，随便用其他的意象也无妨。换成“化为一闪电光”之类的也可以。

“道”自然是量词，但是属于“创意性的用法”。“一道”什么，首先令人想到的就是闪电，这样更加表现了剑之莅飒。在文学中，量词的运用可以带来感官的变化。比如在我所给的句子中“一闪电光”同样用了一个不寻常的字眼来作量词，是一个道理。



This quote looks like it comes from a type of web novel (specifically fantasy stories of martial arts, righteous warriors, and immortal sages).

「惊鸿」refers to a swan goose「鸿」which flies straight upwards (like a missile/jet) whenever it gets startled「惊」, and harks back to the idiom「翩若惊鸿」, which is an old chengyu describing the graceful body figure and mannerisms of immortal beauties.

In this context, the novel has set the usage of the word「惊鸿」. Either this sword being called「惊鸿剑」can really transform into a「惊鸿」, or「惊鸿」is a metaphor for its powers, or「惊鸿」is just one of its powers. Describing the sword as being able to transform into a lightning bolt gives much the same imagery.

「道」is a measure word, but its use here is innovative.「一道X」gives an impression of a flash of lightning, highlighting the swiftness and nimbleness of a sword's motion. In literature, a measure word (that may be different from expected) may be used to bring out some special features of the object measured for the audience. For example, I could use the word「闪」to measure occurrences of lightning.