US Law:
Section 2256 of Title 18, United States Code, defines child pornography as:
any visual depiction of sexually explicit conduct involving a minor (someone under 18 years of age). Visual depictions include photographs, videos, digital or computer generated images indistinguishable from an actual minor, and images created, adapted, or modified, but appear to depict an identifiable, actual minor. Undeveloped film, undeveloped videotape, and electronically stored data that can be converted into a visual image of child pornography are also deemed illegal visual depictions under federal law.
Notably, the legal definition of sexually explicit conduct does not require that an image depict a child engaging in sexual activity. A picture of a naked child may constitute illegal child pornography if it is sufficiently sexually suggestive. Additionally, the age of consent for sexual activity in a given state is irrelevant; any depiction of a minor under 18 years of age engaging in sexually explicit conduct is illegal.
Read more here.
Canadian Law:
"child pornography" means:
(a) a photographic, film, video or other visual representation, whether or not it was made by electronic or mechanical means,
(i) that shows a person who is or is depicted as being under the age of eighteen years and is engaged in or is depicted as engaged in explicit sexual activity, or
(ii) the dominant characteristic of which is the depiction, for a sexual purpose, of a sexual organ or the anal region of a person under the age of eighteen years;
(b) any written material, visual representation or audio recording that advocates or counsels sexual activity with a person under the age of eighteen years that would be an offence under this Act;
(c) any written material whose dominant characteristic is the description, for a sexual purpose, of sexual activity with a person under the age of eighteen years that would be an offence under this Act; or
(d) any audio recording that has as its dominant characteristic the description, presentation or representation, for a sexual purpose, of sexual activity with a person under the age of eighteen years that would be an offence under this Act.
Interpretation Here.