AI Artificial intelligence Publication related to this: Ullah et al. 2022.
Ammonia A compound of nitrogen and hydrogen (NH3). Traditionally produced with Haber-Bosch process. Ammonia is mostly used in the production of nitrogen fertilizers but can be also used in other purposes (e.g. as a fuel). Publications related to this: Galimova et al. 2023, Galimova et al. 2023 and ElSayed et al. 2023. 
Anaerobic digestion Bio-chemical process, which converts organic matter into biogas through degradation. Publication related to this: Ijäs 2022. 
Bioethanol Bioethanol is a fuel produced from biomass. Bioethanol plants are a CO2-source, and the CO2 can be utilized in a biomethanation process. Publication related to this: Ijäs 2022. 
Biogas Consists mainly of methane and carbon dioxide. Biogas is a product of organic matter's anaerobic digestion and is a renewable energy source. Publication related to this: Ijäs 2022.
Biogenic Something that is made by or made of a life form/forms. E.g. trees contain biogenic carbon. Publication related to this: Karjunen et al. 2023.
Blue hydrogen Produced by steam methane reforming (SMR) from natural gas. Carbon dioxide produced in the process is captured with carbon capture and sequestration (CCS) technology. Publication related to this: Patel et al. 2024.
Carbon black Material that is produced in an incomplete combustion process or in thermal decomposition of gaseous or liquid hydrocarbons under controlled conditions. Publication related to this: Patel et al. 2024.
Carbon neutral Carbon emissions and carbon absorption are in balance with each other. 
Carbon price The price of emitted greenhouse gas emissions. A tool for areas and nations to reduce their emissions. 
CCS/CCU Carbon capture and storage/Carbon capture and utilization. Publications related to this: Sillman et al. 2024, Lipiäinen et al. 2023, ElSayed et al. 2023, Karjunen et al. 2023, Nguyen 2022, Patel et al. 2024, and Talus and Smith 2021.
CNG Compressed natural gas
CO2 Carbon dioxide. A well known greenhouse gas. 
CO2-electrolysis In CO2 electrolysis the CO2 is decomposed into carbon products and oxygen. Publication related to this: Aghajanian et al. 2024, Laasonen et al. 2023, and Mäkinen 2022.
CO2-equivalent This metric measure includes all the greenhouse gases of the measured emissions. Greenhouse gases have different global warming potentials (GWPs) and they are converted to be equivalent with the GWP of carbon dioxide. 
e- Electro- (e.g. electrofuel, electrochemical). A synthetic product that is manufactured using CO2 or CO and hydrogen. CO2 or CO is captured and hydrogen is derived using water electrolysis. Publications related to this: Havukainen & Sillman 2024, Lopez et al. 2024, ElSayed et al. 2023, and Galimova et al. 2023.
Electrolysis A direct current passes through an electrolyte which results to a chemical reaction to occur at the electrodes. Due to this chemical reaction, materials decompose. In water electrolysis the water molecule is decomposed into hydrogen and oxygen. Publication related to this: Patel et al. 2024.
Energy carrier A substance, e.g., hydrogen, or phenomenon containing energy. This energy can then be later converted to some other form (e.g., heat or mechanical work).
'Green' In the context of energy, term 'green' refers to renewable energy. For example, green hydrogen is produced using renewable electricity.
Green hydrogen Produced from water through electrolysis using renewable electricity. Publications related to this: Alfasfos et al. 2024, Galimova et al. 2023, J. Räisänen 2023, and R. Kinnunen 2023.
Grey hydrogen Produced by steam methane reforming (SMR) from natural gas. Carbon dioxide is produced as a by-product.
Hydrogen A fuel and an energy carrier which has been proposed to be a promising solution when decarbonising the energy sector. Hydrogen can be produced from different resources, e.g. natural gas, nuclear energy, coal and renewable electricity. Examples of Hygcel publications related to this: Wüstenberg 2023, Talus and Martin 2022, Patel et al. 2024, Sillman et al. 2024, Lipiäinen et al. 2023, Galimova et al. 2023, ElSayed et al. 2023, and Vilve 2023.
In situ Means 'on site' or 'in position'. 
IoT Internet of things Publications related to this: M. Ullah 2023 and Ullah et al. 2022.
LCA Life Cycle Assessment
LNG Liquefied Natural Gas
Methane Methane is a simple hydrocarbon and also the main compound of natural gas and biogas. Methane is also a strong greenhouse gas. Publication related to this: Ijäs 2022. 
Methanol Biofuel which can also be produced synthetically from CO2 and H2. Publications related to this: Karjunen et al. 2023, Nguyen 2022, and Vilve 2023.
Methanation Carbon dioxide and carbon monoxide are converted to methane by hydrogenation. Publications related to this: Lipiäinen et al. 2023 and Ijäs 2022. 
Natural gas Consists mainly of methane, but contains smaller amounts of other hydrocarbons as well. In addition, natural gas usually contains low levels of CO2, N2, H2S, and He. Publications related to this (and/or to natural gas grids in Europe): Lipiäinen et al. 2023, Lipiäinen et al. 2023, Talus 2023, Sillman et al. 2024, Galimova et al. 2023, and Ijäs 2022.
P2G Power-to-gas. Gas (fuel) produced with electricity. 
P2X Power-to-X. For example, fuel or chemical produced with electricity. Publications related to this: Sillman et al. 2024, Karjunen et al. 2023, ElSayed et al. 2023, Vilve 2023, Nguyen 2022, and Räisänen et al. 2022.
PEM Polymer Electrolyte Membrane.
Primary energy Energy found in nature is called primary energy, if it hasn't went through any man made conversion process. There are fossil-based and renewable primary energy sources. 
PV Photovoltaics. The power generation (converting sun light into electricity) is based on the photovoltaic effect. Publications related to this: ElSayed et al. 2023, Galimova et al. 2023, Riaz et al. 2023, and Räisänen et al. 2022. 
RED II Renewable energy directive of the European Union that entered into force in 2018.
Renewable electricity Electricity produced by using renewable energy (wind, solar, etc.).
Sector coupling Energy supply and end-use sectors are integrated to each other. Publications related to this: ElSayed et al. 2023 and Jaanto et al. 2023.
SMR Steam methane reforming. Steam reacts with hydrocarbon producing hydrogen and carbon dioxide. Currently, most of the hydrogen is produced by using SMR. Publication related to this: Patel et al. 2024.
TDM Thermal decomposition of methane. Hydrogen and solid carbon are produced in the process. Publication related to this: Patel et al. 2024.
Turquoise hydrogen Produced by thermal decomposition of methane (natural gas, biogas). Solid carbon is produced as a by-product. Publication related to this: Patel et al. 2024.