No one-word translation. There are several signs (usually compounds) to convey the concept "attest" in contexts.
Meaning: to prove or declare to be true.
E.g. "The fact that you aced the standardized exam attests to the strength of your reading skills.
Meaning: to show something or to say or prove that something is true. E.g. Thousands of Deaf people and hearing allies came out onto the streets to attest their support for the official recognition of ASL in their country.
Meaning: declare that something exists or is the case.
Meaning (law): to sign a document to show that you were present when it was signed by someone else. E.g. The will must be attested by two witnesses.
Meaning: to say officially that something is true or accurate.
Search/Filter: Enter a keyword in the filter/search box to see a list of available words with the "All" selection. Click on the page number if needed. Click on the blue link to look up the word. For best result, enter a partial word to see variations of the word.
Alphabetical letters: It's useful for 1) a single-letter word (such as A, B, etc.) and 2) very short words (e.g. "to", "he", etc.) to narrow down the words and pages in the list.
For best result, enter a short word in the search box, then select the alphetical letter (and page number if needed), and click on the blue link.
Don't forget to click "All" back when you search another word with a different initial letter.
If you cannot find (perhaps overlook) a word but you can still see a list of links, then keep looking until the links disappear! Sharpening your eye or maybe refine your alphabetical index skill. :)
Add a Word: This dictionary is not exhaustive; ASL signs are constantly added to the dictionary. If you don't find a word/sign, you can send your request (only if a single link doesn't show in the result).
Videos: The first video may be NOT the answer you're looking for. There are several signs for different meanings, contexts, and/or variations. Browsing all the way down to the next search box is highly recommended.
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Grammar: ASL has its own grammar and structure in sentences that works differently from English. For plurals, verb inflections, word order, etc., learn grammar in the "ASL Learn" section. For search in the dictionary, use the present-time verbs and base words. If you look for "said", look up the word "say". Likewise, if you look for an adjective word, try the noun or vice versa. E.g. The ASL signs for French and France are the same. If you look for a plural word, use a singular word.
Inflection: Many ASL words, especially verbs, in the dictionary are a "base"; be aware that many of them are grammatically inflectable within ASL sentences. Some entries have sentence examples.
Variation: Some ASL signs have regional (and generational) variations across North America. Some common variations are included as much as possible, but for specifically local variations, interact with your local community to learn their local variations.
Contextual meaning: Some ASL signs in the dictionary may not mean the same in different contexts and/or ASL sentences. You will see some examples in video sentences.
ASL is very much alive and indefinitely constructable as any spoken language. The best way to use ASL right is to immerse in daily language interactions and conversations with Deaf/Ameslan people (or ASLians).