Definition of theta in English:


(also θ)

Pronunciation /ˈTHādə/ /ˈθeɪdə/ /ˈTHēdə/ /ˈθidə/


  • 1The eighth letter of the Greek alphabet (Θ, θ), transliterated as ‘th.’.

    ‘English borrowed the digraph th from Latin, where it served to transliterate Greek theta; th superseded other symbols for the dental fricative following the advent of printing.’
    • ‘Look lower down the alphabet - theta perhaps, or even upsilon.’
    • ‘With these basic relationships in mind, let's now take a closer look at time value and the rate of time-value decay (represented by theta, from the Greek alphabet).’
    1. 1.1ThetaAstronomy followed by Latin genitive The eighth star in a constellation.
      ‘Theta Draconis’
      • ‘It is 4.8° SSW of Theta Aurigae.’
      • ‘Some of Ara's stars are very distant: both Gamma Arae and Theta Arae lie more than 1,000 light years away, but both of these stars are brilliant supergiants, and so are clearly visible to the naked eye of an observer on Earth.’
    2. 1.2Chemistry as modifier Denoting a temperature at which a polymer solution behaves ideally as regards its osmotic pressure.
      ‘For a given star polymer the theta temperature can be lower or higher than that of the corresponding linear polymer depending on the number and length of its branches.’
      • ‘We use equations derived from the blob theory to calculate the blob size and the theta temperature of star polymers.’
    3. 1.3as modifier Denoting electrical activity observed in the brain under certain conditions, consisting of oscillations having a frequency of 4 to 7 hertz.
      ‘theta rhythm’
      • ‘Abnormalities in electroencephalograph theta and delta activity, visual evoked potentials, and brainstem evoked potentials have been reported in children exposed to molds.’
      • ‘The abnormalities have included right hemisphere dominance and excessively slow frequency EEG theta activity.’
      • ‘During the transition from a waking state to a drowsy-sleep state, the participant might lose the dominant beta/alpha frequencies and display theta activity.’
      • ‘In contrast to results for sigma and delta frequencies, no strong differences between sleepy and less sleepy subjects were noted in theta, alpha, and beta ranges.’
      • ‘Look up ‘beta, alpha, theta and delta’ brain states on the internet.’
      • ‘By contrast, drowsiness and light sleep are characterized by lower-frequency theta activity.’
      • ‘However, the present data together with those previously published for respiratory evoked potentials indicate that the movement from alpha to theta EEG activity represents a substantial change in neural responsiveness.’
      • ‘Slower frequency 4-7 Hz theta activity begins to emerge, and slow horizontal eye movements may also be apparent.’
      • ‘Activity in the alpha range is generally of a much higher amplitude than that in the theta or beta range.’
      • ‘Does a fetus attain an alpha/beta/theta/delta brainwave state?’
      • ‘Age-related cognitive disorders are typically associated with increases in the delta, theta, and fast beta ranges, while the alpha and slow beta frequencies decrease.’
      • ‘Hypnotherapy is one of the tools I use because it puts people in alpha and theta brainwave states; highly receptive levels of consciousness where suggestibility is at its maximum.’
      • ‘First my head was wrapped to monitor my alpha and theta brainwaves.’
      • ‘Very deep relaxation and meditation produce a pattern that combines the so-called alpha and theta rhythms, indicating a state of harmony.’

  • 1Temperature (especially in degrees Celsius).

  • 2A plane angle.

  • 3A polar coordinate.

    Often coupled with phi