ortho- Look up ortho- at Dictionary.com
before vowels orth-, word-forming element meaning "straight, upright, rectangular, regular; true, correct, proper," now mostly in scientific and technical compounds, from Greek ortho-, stem of orthos "straight, true, correct, regular," from PIE *eredh- "high" (source also of Sanskrit urdhvah "high, lofty, steep," Latin arduus "high, steep," Old Irish ard "high").