What you will see below, is an extract from Microsoft Academic Graph. I counted every keyword used in more than 80 million research papers.
- humanidades - 720,429
- bioinformatics - 700,032
- [biomedical research](https://academic.microsoft.com/#/search?q=biomedical research&iq=%2540biomedical research%2540&filters=&from=0&sort=0) - 624,874
- kinetics - 449,695
- genetics - 438,081
- spectrum - 427,706
- biology - 358,565
- medicine - 356,349
- [mathematical model](https://academic.microsoft.com/#/search?q=mathematical model&iq=%2540mathematical model%2540&filters=&from=0&sort=0) - 338,009
- [magnetic field](https://academic.microsoft.com/#/search?q=magnetic field&iq=%2540magnetic field%2540&filters=&from=0&sort=0) - 335,559
- enzyme - 309,175
- nitrogen - 300,273
- iron - 299,830
- [cross section](https://academic.microsoft.com/#/search?q=cross section&iq=%2540cross section%2540&filters=&from=0&sort=0) - 295,181
- oscillations - 288,823
- [higher education](https://academic.microsoft.com/#/search?q=higher education&iq=%2540higher education%2540&filters=&from=0&sort=0) - 280,800
- [human factors](https://academic.microsoft.com/#/search?q=human factors&iq=%2540human factors%2540&filters=&from=0&sort=0) - 269,558
- copper - 261,244
- indexation - 261,012
- [three dimensional](https://academic.microsoft.com/#/search?q=three dimensional&iq=%2540three dimensional%2540&filters=&from=0&sort=0) - 251,813
- ergonomics - 251,674
- [occupational safety](https://academic.microsoft.com/#/search?q=occupational safety&iq=%2540occupational safety%2540&filters=&from=0&sort=0) - 247,894
- [injury prevention](https://academic.microsoft.com/#/search?q=injury prevention&iq=%2540injury prevention%2540&filters=&from=0&sort=0) - 246,608
- [suicide prevention](https://academic.microsoft.com/#/search?q=suicide prevention&iq=%2540suicide prevention%2540&filters=&from=0&sort=0) - 244,970
- [thin film](https://academic.microsoft.com/#/search?q=thin film&iq=%2540thin film%2540&filters=&from=0&sort=0) - 235,267
- physics - 233,603
- history - 233,036
- management - 229,268
- health - 218,187
- cancer - 208,652
- [risk factors](https://academic.microsoft.com/#/search?q=risk factors&iq=%2540risk factors%2540&filters=&from=0&sort=0) - 207,986
- [x ray diffraction](https://academic.microsoft.com/#/search?q=x ray diffraction&iq=%2540x ray diffraction%2540&filters=&from=0&sort=0) - 205,406
- [crystal structure](https://academic.microsoft.com/#/search?q=crystal structure&iq=%2540crystal structure%2540&filters=&from=0&sort=0) - 204,879
- satisfiability - 200,346
- mathematics - 196,977
- thermodynamics - 195,849
- biotechnology - 193,032
- temperature - 191,481
- chemistry - 191,432
- [gene expression](https://academic.microsoft.com/#/search?q=gene expression&iq=%2540gene expression%2540&filters=&from=0&sort=0) - 185,834
- [room temperature](https://academic.microsoft.com/#/search?q=room temperature&iq=%2540room temperature%2540&filters=&from=0&sort=0) - 183,766
- environment - 181,565
- [real time](https://academic.microsoft.com/#/search?q=real time&iq=%2540real time%2540&filters=&from=0&sort=0) - 181,021
- microstructures - 179,956
- [public health](https://academic.microsoft.com/#/search?q=public health&iq=%2540public health%2540&filters=&from=0&sort=0) - 179,075
- publishing - 178,056
- [high resolution](https://academic.microsoft.com/#/search?q=high resolution&iq=%2540high resolution%2540&filters=&from=0&sort=0) - 177,266
- [cell line](https://academic.microsoft.com/#/search?q=cell line&iq=%2540cell line%2540&filters=&from=0&sort=0) - 175,158
- simulation - 174,505
- engineering - 172,942
- internet - 172,592
- apoptosis - 171,952
- agriculture - 170,323
- [amino acid](https://academic.microsoft.com/#/search?q=amino acid&iq=%2540amino acid%2540&filters=&from=0&sort=0) - 169,489
- infrared - 169,464
- obesity - 167,196
- epidemiology - 166,894
- [steady state](https://academic.microsoft.com/#/search?q=steady state&iq=%2540steady state%2540&filters=&from=0&sort=0) - 166,661
- [chemical reaction](https://academic.microsoft.com/#/search?q=chemical reaction&iq=%2540chemical reaction%2540&filters=&from=0&sort=0) - 166,113
- [climate change](https://academic.microsoft.com/#/search?q=climate change&iq=%2540climate change%2540&filters=&from=0&sort=0) - 165,297
- statistics - 164,992
- seasonality - 164,331
- [numerical simulation](https://academic.microsoft.com/#/search?q=numerical simulation&iq=%2540numerical simulation%2540&filters=&from=0&sort=0) - 160,715
- [materials science](https://academic.microsoft.com/#/search?q=materials science&iq=%2540materials science%2540&filters=&from=0&sort=0) - 159,503
- [electric field](https://academic.microsoft.com/#/search?q=electric field&iq=%2540electric field%2540&filters=&from=0&sort=0) - 159,212
- testing - 158,349
- [statistical analysis](https://academic.microsoft.com/#/search?q=statistical analysis&iq=%2540statistical analysis%2540&filters=&from=0&sort=0) - 152,828
- [computer simulation](https://academic.microsoft.com/#/search?q=computer simulation&iq=%2540computer simulation%2540&filters=&from=0&sort=0) - 152,742
- [cell cycle](https://academic.microsoft.com/#/search?q=cell cycle&iq=%2540cell cycle%2540&filters=&from=0&sort=0) - 151,380
- [single crystal](https://academic.microsoft.com/#/search?q=single crystal&iq=%2540single crystal%2540&filters=&from=0&sort=0) - 151,208
- optimization - 151,145
- nickel - 150,057
- [physical properties](https://academic.microsoft.com/#/search?q=physical properties&iq=%2540physical properties%2540&filters=&from=0&sort=0) - 150,031
- [signal transduction](https://academic.microsoft.com/#/search?q=signal transduction&iq=%2540signal transduction%2540&filters=&from=0&sort=0) - 149,892
- design - 148,610
- [data mining](https://academic.microsoft.com/#/search?q=data mining&iq=%2540data mining%2540&filters=&from=0&sort=0) - 148,020
- [mathematical models](https://academic.microsoft.com/#/search?q=mathematical models&iq=%2540mathematical models%2540&filters=&from=0&sort=0) - 147,438
- [chemical composition](https://academic.microsoft.com/#/search?q=chemical composition&iq=%2540chemical composition%2540&filters=&from=0&sort=0) - 147,438
- [escherichia coli](https://academic.microsoft.com/#/search?q=escherichia coli&iq=%2540escherichia coli%2540&filters=&from=0&sort=0) - 146,743
- zinc - 143,014
- [quantum mechanics](https://academic.microsoft.com/#/search?q=quantum mechanics&iq=%2540quantum mechanics%2540&filters=&from=0&sort=0) - 142,616
- ecology - 137,861
- economics - 136,692
- [heat transfer](https://academic.microsoft.com/#/search?q=heat transfer&iq=%2540heat transfer%2540&filters=&from=0&sort=0) - 133,496
- stability - 132,877
- [breast cancer](https://academic.microsoft.com/#/search?q=breast cancer&iq=%2540breast cancer%2540&filters=&from=0&sort=0) - 132,851
- dna - 132,459
- algorithms - 131,587
- potassium - 131,297
- [data collection](https://academic.microsoft.com/#/search?q=data collection&iq=%2540data collection%2540&filters=&from=0&sort=0) - 129,991
- frequency - 129,881
- immunology - 127,795
- [teaching methods](https://academic.microsoft.com/#/search?q=teaching methods&iq=%2540teaching methods%2540&filters=&from=0&sort=0) - 127,142
- [remote sensing](https://academic.microsoft.com/#/search?q=remote sensing&iq=%2540remote sensing%2540&filters=&from=0&sort=0) - 126,897
- phosphorus - 125,845
- veterinary - 123,708
- polymorphism - 123,335
- [elementary particles](https://academic.microsoft.com/#/search?q=elementary particles&iq=%2540elementary particles%2540&filters=&from=0&sort=0) - 123,305
- commissioning - 123,260
- [academic research](https://academic.microsoft.com/#/search?q=academic research&iq=%2540academic research%2540&filters=&from=0&sort=0) - 122,516