Definition of theta in US English:



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

    • ‘Look lower down the alphabet - theta perhaps, or even upsilon.’
    • ‘English borrowed the digraph th from Latin, where it served to transliterate Greek theta; th superseded other symbols for the dental fricative following the advent of printing.’
    • ‘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.
      • ‘We use equations derived from the blob theory to calculate the blob size and the theta temperature of star polymers.’
      • ‘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.’
    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’
      • ‘Activity in the alpha range is generally of a much higher amplitude than that in the theta or beta range.’
      • ‘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.’
      • ‘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.’
      • ‘First my head was wrapped to monitor my alpha and theta brainwaves.’
      • ‘By contrast, drowsiness and light sleep are characterized by lower-frequency theta activity.’
      • ‘The abnormalities have included right hemisphere dominance and excessively slow frequency EEG theta activity.’
      • ‘Slower frequency 4-7 Hz theta activity begins to emerge, and slow horizontal eye movements may also be apparent.’
      • ‘Look up ‘beta, alpha, theta and delta’ brain states on the internet.’
      • ‘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.’
      • ‘Very deep relaxation and meditation produce a pattern that combines the so-called alpha and theta rhythms, indicating a state of harmony.’
      • ‘Abnormalities in electroencephalograph theta and delta activity, visual evoked potentials, and brainstem evoked potentials have been reported in children exposed to molds.’
      • ‘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.’
      • ‘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.’
      • ‘Does a fetus attain an alpha/beta/theta/delta brainwave state?’

  • 1Temperature (especially in degrees Celsius).

  • 2A plane angle.

  • 3A polar coordinate.

    Often coupled with phi