EDIT: A comment made me realize that I should probably mention I'm approaching this from an angle of their being a singular legitimate truth and that consensus reality is our best working model of that truth.
Necessarily everything we perceive must pass through our senses in one way or another. Likewise, any learning must also necessarily come from a source that is similar compromised but also has taken certain stances on uncertain issues or has taken no stances for the purposes of education at all.
Is it possible to be truly unbiased?
So this means:
In some way, all our thoughts and perceptions differ at least slightly from consensus reality.
We can never take anything as truth with absolutele certainty and should make every effort to be aware of possible biases in consideration of truth.
When is it a lie:
It is a lie if some failure on the part of the sharer results in a greater deviance from truth than absolutely necessary. Under some models, this failure must be intentionally.
What is a lie:
A lie is something presented as truth that, for whatever reason (under some models necessarily malice) presents either more bias or less decisiveness than the minimum. Under some models, lies may be acceptable within a range of the minimum rather than at a hard minimum.
Let's do an example:
I perceive the universe from a monotheistic viewpoint. While in my presentation to others I attempt to be clear about my biases, necessarily I cannot present them completely.
Perhaps I am speaking with a polytheist.
A polytheist speaking with me may place unreasonable credibility on my beliefs as I will necessarily present them as less biased than they are. However, this is as close to truth as myself and the polytheist may come. However, we must both understand that there will necessarily some deviation from truth.
A lie would be me positing my opinions (or any opinions really) as truth without making clear my biases.
This would be a lie because it presents an unfairly high degree of accuracy and does not have certainty in its adherence to truth.
I hope that helps.