if the definitions are the same (pretty much) is this gonna look bad in terms of copyright

I doubt it. Just make sure you don't crib the definitions word for word from your competitor's sites.

and or search engines

Yes.

If your definitions ARE just about the same as on any other site (and if you are providing accurate, industry-standard definitions, they will be) this is as to be expected.

Get over it.

In other words, you can't SEO-optimize every aspect of your site. Some parts of it might actually serve the needs of your users.

You need basic real-estate definitions for the benefit of your users. It's not very sexy or glamorous, nor likely to stand-out from anybody else's definitions. But it's something you have to have there - like the magazines racks at a magazine stand. But don't expect more from it - people aren't going to come from miles around because you have the finest magazine racks!

In fact, if you really have the needs of your users in mind, you might consider sending them off to (or partnering with) some other (non-competing) site that has excellent real-estate definitions.