What is wrong speech according to Buddhism?

If you say for example, "from what I know:...", or, "I have heard that:...", then there would be no lie (at worst it's useless chatter). If you say it as f you really know for example, "it is...", "he has...", "that is wrong...", with definite speech -- and if happens that your assumptions turn out to be wrong, you understood wrong -- then you did not really know, and that it was simply a lie.

If you speak in ways of truthfulness, it's not possible to lie.

Think on that, if saying, "tomorrow I will do this and that...", or in ways of, "I am...", and of how many lies are around on places like this?

One who says "i know" but does not; "I see" but does not; "I have heard" but hasn't; is called a liar by those who are serious.

And one should not forget that, to gain liberation, it requires to abandon all kinds of stealing the truth; and untruthfulness starts with stopping "speaking of what is not true."

For the most speech is to defend one's world, identification and it would require freedom from stinginess (e.g. Noble person) to have not simply just this intention, however excused or hidden.

One may think on Rahula's first lesson he got. There is no way in Dhamma for one who does not start right here; and even the Bodhisatta, although still (before enlightenment) breaking this or that precept, never failed in sacca, truthfulness. Truthfulness as well as the Buddha-Dhamma is not for everyone, not to speak about careless.

Yet think then on the even more fine matter of changing ones mind, and it's impact.

Much liberating gain in going truthfully into this matter, rather to seek approve under equal and lower with the defiled "I had the intention" excuse (yet not even recognize the drive).

[Text is not placed for trade or to be used for any wordily exchange]

In addition to the answers provided, to be safe, if you were unsure of something you have to indicate that you were unsure of it. Same for if you don't know, say you don't know. If it is not something you personally verified, make sure you indicate that as well.

Especially if the topic in question was dire or can have a big impact to others based on your answers, I would advise you be very careful of giving the right answers because not only it would bring impact to others, but also risky in committing false speech.

In the broader context of ignoble speech, we have DN33:

Four ignoble expressions: speech that’s false, divisive, harsh, or nonsensical.

Notice that lying is included in the first of these. Lying is intentional. One states what is false. You had no intention of lying but your speech was false. And you felt doubt and perhaps remorse later.

To blurt out something baldly and assertively is dangerous as you've found yourself later that the statement was false. Rather than hinder yourself with remorse (part of the five hindrances), simply be mindful that as the name and forms we use in speech come into being, they also must mutate and cease. Perhaps the simplest mindful thing to do is prefix statements (especially any "obvious truths") with "it is said", "i have heard", or other qualifying phrases. The effort of doing so will gradually pay off and help mindfulness grow.

And from DN33 as well:

refraining from speech that’s false, divisive, harsh, or nonsensical

Which also means that saying nothing at all is also fine.

🙏

I don't think the fourth precept is defined in detailed in suttas. Actually I don't think any of the precepts are defined in detail -- perhaps they are in a commentary.

Something like that precept, though, is defined in the vinaya (for monks) -- see Chapter 8, page 298, of The Pāṭimokkha Rules Translated & Explained by Ṭhānissaro Bhikkhu, which includes:

1. Intention: the aim to misrepresent the truth; and
2. Effort: the effort to make another individual know whatever one wants to communicate based on that aim.

But I don't understand your question saying, "I just said what I know and I found out later that it was false" -- because, how could you "know" something that's false?

I think (I hope) I'd tend to phrase that as, "I think that so-and-so", or "Someone once told me so-and-so", or "I guess that so-and-so", or "Such-and-such a reference says so-and-so" -- not, "I know that so-and-so is true" when I'm not sure.

If I'm not sure about something, then conveying more certainty than I have (e.g. by saying, "X is definitely true") is in my opinion misrepresenting the truth about how good my knowledge is, how much certainty I have, how trust-worthy it is.

Anyway you should probably read that chapter of the vinaya, it includes various details which I won't quote here.

The vinaya says it's wrong to break a promise, too. In my opinion (though, I'd guess, not according to Buddhist scripture) that means you should avoid kind of implying (perhaps an implied promise) that something is true when it isn't -- so for example if you're not certain then don't imply you're certain.

Since you mentioned "wrong speech" you might also want to read about "right speech" more generally, i.e. one of the factors of the noble eightfold path (I don't know whether that's exactly the same as the fourth precept):

• Right Speech (quotes from Suttas)
• Right Speech (a little essay by Thanissaro Bhikkhu)