Definition of ortho- in English:

ortho-

Pronunciation: /ˈɔːθəʊ/

combining form

2 Chemistry Denoting substitution at two adjacent carbon atoms in a benzene ring, e.g. in 1,2 positions: orthodichlorobenzene Compare with meta- and para-1.
3 Chemistry Denoting a compound from which a meta-compound is formed by dehydration: orthophosphoric acid

Origin

from Greek orthos 'straight, right'.

Definition of ortho- in:

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Word of the day demoralize
Pronunciation: dəˈmôrəˌlīz
verb
cause (someone) to lose confidence or hope; dispirit