1. They are

Sounds like: their, there

• tertian
• tertiary
• Tertiary period
• tertiary syphilis
• tertigravida
• tertium quid
• Tertry
• Tertullian
• Terylene
• terza rima
• terzetto
• TESL
• tesla
• Tesla coil
• TESOL
• tessella
• tessellate
• tessellated
• tessellation
• tessera
• tesseract
• Tessin
• tessitura
• test
• test ban