"document is not defined" in Nuxt.js

It's a common error when you start a Nuxt project ;-)

The Choices.js lib is available only for client-side! So Nuxt tried to renderer from server-side, but from Node.js window.document doesn't exist, then you have an error.
nb: window.document is only available from the browser renderer.

Since Nuxt 1.0.0 RC7, you can use <no-ssr> element to allow your component only for client-side.

<template>
  <div>
    <no-ssr placeholder="loading...">
      <your-component>
    </no-ssr>
  </div>
</template>

take a look at the official example here: https://github.com/nuxt/nuxt.js/blob/dev/examples/no-ssr/pages/index.vue

---

Update:

Since Nuxt >= 2.9.0, you have to use the <client-only> element instead of <no-ssr>:

<template>
  <div>
    <client-only placeholder="loading...">
      <your-component>
    </client-only>
  </div>
</template>

To know more, see nuxt docs: https://nuxtjs.org/docs/2.x/features/nuxt-components#the-client-only-component

The accepted answer (while correct) was too short for me to understand it and use it correctly, so I wrote a more detailed version. I was looking for a way to use plotly.js + nuxt.js, but it should be the same as the OP's problem of Choice.js + nuxt.js.

MyComponent.vue

<template>
  <div>
    <client-only>
      <my-chart></my-chart>
    </client-only>
  </div>
</template>
<script>
export default {
  components: {
    // this different (webpack) import did the trick together with <no-ssr>:
    'my-chart': () => import('@/components/MyChart.vue')
  }
}
</script>

MyChart.vue

<template>
  <div>
  </div>
</template>
<script>
import Plotly from 'plotly.js/dist/plotly'
export default {
  mounted () {
    // exists only on client:
    console.log(Plotly)
  },
  components: {
    Plotly
  }
}
</script>

Update: There is <client-only> tag instead of <<no-ssr> in Nuxt v>2.9.0, see @Kaz's comment.