"theta wave" meaning in English

{
  "etymology_templates": [
    {
      "args": {
        "1": "en",
        "2": "theta",
        "3": "wave"
      },
      "expansion": "theta + wave",
      "name": "compound"
    }
  ],
  "etymology_text": "theta + wave",
  "forms": [
    {
      "form": "theta waves",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "theta wave (plural theta waves)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "A type of electrical activity in the brain; a brain wave with a frequency of 4-8 hertz."
      ],
      "id": "en-theta_wave-en-noun-BUxrR7gO",
      "links": [
        [
          "brain wave",
          "brain wave"
        ],
        [
          "hertz",
          "hertz"
        ]
      ],
      "related": [
        {
          "word": "theta rhythm"
        }
      ],
      "translations": [
        {
          "code": "es",
          "lang": "Spanish",
          "sense": "electrical activity in the brain",
          "tags": [
            "feminine"
          ],
          "word": "onda theta"
        }
      ],
      "wikipedia": [
        "theta wave"
      ]
    }
  ],
  "word": "theta wave"
}

{
  "etymology_templates": [
    {
      "args": {
        "1": "en",
        "2": "theta",
        "3": "wave"
      },
      "expansion": "theta + wave",
      "name": "compound"
    }
  ],
  "etymology_text": "theta + wave",
  "forms": [
    {
      "form": "theta waves",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "theta wave (plural theta waves)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "theta rhythm"
    }
  ],
  "senses": [
    {
      "categories": [
        "English compound terms",
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns"
      ],
      "glosses": [
        "A type of electrical activity in the brain; a brain wave with a frequency of 4-8 hertz."
      ],
      "links": [
        [
          "brain wave",
          "brain wave"
        ],
        [
          "hertz",
          "hertz"
        ]
      ],
      "wikipedia": [
        "theta wave"
      ]
    }
  ],
  "translations": [
    {
      "code": "es",
      "lang": "Spanish",
      "sense": "electrical activity in the brain",
      "tags": [
        "feminine"
      ],
      "word": "onda theta"
    }
  ],
  "word": "theta wave"
}