Speech

Speech is a Fallout, Fallout 2, and Fallout 3 skill. In Van Buren, it was divided into two skills - Persuasion and Deception.

The ability to communicate in a practical and efficient manner. The skill of convincing others that your position is correct. The ability to lie and not get caught.

Fallout and Fallout 2
Initial Level: Starting Speech skill is equal to 25% + (2% x Charisma). Average characters will have a 35% skill.

Speech skill was in the first draft of Fallout Tactics skills, but was eventually left out.

Fallout 3
Occasionally, you will be presented with a Speech Challenge. A Speech Challenge is a response preceded by the word “Speech” and the percentage chance of success (e.g., [Speech 25%]); the higher your Speech skill, the greater the chance of success.

Succeed in a Speech Challenge, and you’ll achieve the results indicated by the response. Fail a Speech Challenge, and future Speech Challenges with that character will be more difficult.

From time to time you may also be presented with dialogue options where a word appears with no percentage (e.g., Strength or Lady Killer). In this case, this is a dialogue option that is available to you because of the level of the indicated S.P.E.C.I.A.L. statistic, Skill, or Perk and you will always be successful when selecting it. You only get one chance at each Speech Challenge, so if encounter one has a low percentage to succeed, you may want to exit dialogue and try the Speech challenge at a later time, after you’ve raised your Speech skill.

A permanent +10 to your speech skill can be earned as a possible reward from the quest Oasis, meaning that you do not need to level Speech above level 80, as the final 20 points can be obtained as reward from aformentioned quest, in addition to the Bobblehead.

Fan Edit: If you go to Big Town with a high enough speech level (80%+) and speak to Pappy (if he hasn't died during the mutant or raider raids yet) you'll be presented with the option of asking him about his relationship with Bittercup. This speech challenge can be repeated for as many times as necessary to get yourself the Silver-tongued Devil achievement (but you wouldn't want to be cheap, would you?)