is this word both 'the force of gravity' and 'grave seriousness'?
Thank you shwmae, that's a big help. I can't seem to get on with that Geiriadur dictionary, so it's nice to be able to shout for help.
Geiriadur yr Academi is really for pretty advanced students. A better day-to-day one online is Geiriadur Bangor: http://geiriadur.bangor.ac.uk/ It can be downloaded as a free app too: https://www.bangor.ac.uk/canolfanbedwyr/ap_geiriaduron.php.en
@Hatakend Dim problem. That site looks like it just goes through Google and Bing Translate, rather than being a source of reliable dictionary entries. That's fine, but just so that you're aware, those sites aren't always a reliable source of good modern translations. But I'd agree, with that proviso, use what works for you. Another site is http://www.gweiadur.com/en/Pawb. Pob hwyl / All the best!
The Bangor dictionary does seem less daunting, but giving instructions only in Welsh just feels unfriendly.